Permanent Prostate Seed Implant Brachytherapy (Reprinted from Medical Physics, Vol. 26, Issue 10)

Permanent Prostate Seed Implant Brachytherapy (Reprinted from Medical Physics, Vol. 26, Issue 10)
Permanent prostate seed implant brachytherapy: Report of the American
Association of Physicists in Medicine Task Group No. 64a…
Yan Yub)
Department of Radiation Oncology, University of Rochester, Rochester, New York 14642
Lowell L. Anderson
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York 10021
Zuofeng Li
Department of Radiation Oncology, University of Florida, Gainesville, Florida 32610
David E. Mellenberg
Radiation Oncology Division, University of Iowa, Iowa City, Iowa 52242
Ravinder Nath
Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, Connecticut 06510
M. C. Schell
Department of Radiation Oncology, University of Rochester, Rochester, New York 14642
Frank M. Waterman
Department of Radiation Oncology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107
Andrew Wu
Department of Radiation Oncology, Allegheny General Hospital, Pittsburgh, Pennsylvania 15212
John C. Blasko
University of Washington Medical Center, Seattle, Washington 98195
共Received 28 April 1999; accepted for publication 20 July 1999兲
There is now considerable evidence to suggest that technical innovations, 3D image-based planning, template guidance, computerized dosimetry analysis and improved quality assurance practice
have converged in synergy in modern prostate brachytherapy, which promise to lead to increased
tumor control and decreased toxicity. A substantial part of the medical physicist’s contribution to
this multi-disciplinary modality has a direct impact on the factors that may singly or jointly determine the treatment outcome. It is therefore of paramount importance for the medical physics
community to establish a uniform standard of practice for prostate brachytherapy physics, so that
the therapeutic potential of the modality can be maximally and consistently realized in the wider
healthcare community. A recent survey in the U.S. for prostate brachytherapy revealed alarming
variance in the pattern of practice in physics and dosimetry, particularly in regard to dose calculation, seed assay and time/method of postimplant imaging. Because of the large number of start-up
programs at this time, it is essential that the roles and responsibilities of the medical physicist be
clearly defined, consistent with the pivotal nature of the clinical physics component in assuring the
ultimate success of prostate brachytherapy. It was against this background that the Radiation
Therapy Committee of the American Association of Physicists in Medicine formed Task Group No.
64, which was charged 共1兲 to review the current techniques in prostate seed implant brachytherapy,
共2兲 to summarize the present knowledge in treatment planning, dose specification and reporting, 共3兲
to recommend practical guidelines for the clinical medical physicist, and 共4兲 to identify issues for
future investigation. © 1999 American Association of Physicists in Medicine.
关S0094-2405共99兲00910-4兴
Key words: brachytherapy, interstitial brachytherapy, prostate seed implant, quality assurance,
standards of practice
TABLE OF CONTENTS
GLOSSARY OF ABBREVIATIONS . . . . . . . . . . . . .
I. INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. REVIEW OF CURRENT TECHNIQUES. . . . . . . .
A. Overview of contemporary techniques. . . . . . . .
B. Treatment planning techniques. . . . . . . . . . . . . .
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1.
2.
3.
4.
5.
6.
7.
Ultrasound volume study. . . . . . . . . . . . . . . .
Pubic arch. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seed distribution. . . . . . . . . . . . . . . . . . . . . . .
Urethra. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rectum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dose margin. . . . . . . . . . . . . . . . . . . . . . . . . .
Intraoperative planning. . . . . . . . . . . . . . . . . .
0094-2405/99/26„10…/2054/23/$15.00
© 1999 Am. Assoc. Phys. Med.
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C.
D.
E.
F.
Equipment and applicators. . . . . . . . . . . . . . . . . .
Source type, assay and preparation. . . . . . . . . . .
Implantation procedure. . . . . . . . . . . . . . . . . . . . .
Postimplant dosimetry. . . . . . . . . . . . . . . . . . . . .
1. Rationale. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Technical issues. . . . . . . . . . . . . . . . . . . . . . .
III. REVIEW OF DOSIMETRIC ASPECTS. . . . . . . .
A. Historical perspective on dosimetry. . . . . . . . . .
B. Dosimetry data revision. . . . . . . . . . . . . . . . . . . .
C. Dose specification and reporting. . . . . . . . . . . . .
D. Dosimetric uncertainties. . . . . . . . . . . . . . . . . . . .
E. Treatment plan evaluation. . . . . . . . . . . . . . . . . .
1. S mPD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Dose uniformity. . . . . . . . . . . . . . . . . . . . . . .
3. Dose conformity. . . . . . . . . . . . . . . . . . . . . . .
4. Dose-volume histogram. . . . . . . . . . . . . . . . .
F. Treatment plan optimization. . . . . . . . . . . . . . . .
IV. CURRENT RECOMMENDATIONS. . . . . . . . . . .
A. Equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. New radionuclide designs. . . . . . . . . . . . . . . . . .
C. Seed assay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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E.
F.
G.
H.
I.
J.
Changes to the dose value due to TG43. . . . . . .
Dosimetric planning. . . . . . . . . . . . . . . . . . . . . . .
Implantation procedure. . . . . . . . . . . . . . . . . . . . .
Patient release. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Postimplant analysis. . . . . . . . . . . . . . . . . . . . . . .
Training requirement for physics personnel. . . .
Recommendations regarding commercial
treatment planning systems. . . . . . . . . . . . . . . . .
V. ISSUES FOR FUTURE CONSIDERATION. . . . .
A. Anisotropic dose calculation. . . . . . . . . . . . . . . .
B. Interseed effect. . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Tissue heterogeneity. . . . . . . . . . . . . . . . . . . . . . .
D. Biological models. . . . . . . . . . . . . . . . . . . . . . . . .
E. Relative biological effectiveness. . . . . . . . . . . . .
F. Time course of target volume change. . . . . . . . .
G. Differential dose planning and delivery. . . . . . .
H. Intraoperative seed localization and
dosimetry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I. Correlation of dosimetric and clinical
outcomes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . .
GLOSSARY OF ABBREVIATIONS
ADCL
D 100 ,D 90 ,D 80
DVH
Gleason score
MPD
mPD
Accredited
Dosimetry
Calibration
Laboratory.
Dose to 100%, 90%, 80% of the target
volume for dosimetric evaluation.
Dose-volume histogram.
A pathological grading system for measuring the degree of differentiation of
prostate tumors.
Matched peripheral dose, typically used
in conjunction with the ellipsoidal approximation for the prostate volume.
Minimum peripheral dose. Used in
dosimetric planning, the mPD corresponds to the isodose surface that just
encompasses the planning target volume. This report recognizes that
I. INTRODUCTION
Adenocarcinoma of the prostate is the most common malignancy in man in the United States, excluding skin cancer.
The American Cancer Society estimated that 184,500 new
cases of prostate cancer would be diagnosed in the U.S. in
1998. Growing emphasis on prostatic specific antigen 共PSA兲
based early detection and changes in the population demographics in the U.S. suggest that the number of newly diagnosed cases would continue to increase each year. In the
treatment of prostate cancer, there is now a broad resurgence
of interest in the role of permanent interstitial implantation of
radioactive seeds. Prostate seed implants are currently perMedical Physics, Vol. 26, No. 10, October 1999
NIST
PSA
PTV
S mPD
TRUS
V 200 ,V 100 ,V 90 ,V 80
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whereas the mPD usually corresponds
to a prescribed dose, D 100 as determined
from postimplant dosimetry may be significantly different from the prescribed
dose.
National Institute of Standards and
Technology.
Prostate specific antigen.
Planning target volume.
Total source strength required to
achieve 1 Gy in the mPD, typically used
in dosimetric planning and optimization.
Transrectal ultrasound.
共fractional兲 Volume of the prostate target for dosimetric evaluation that received 200%, 100%, 90%, 80% of the
prescribed mPD.
formed using iodine-125 and palladium-103 sources under
imaging and template guidance to deliver localized irradiation to high doses. For selected patients, seed implantation
alone offers a complete course of treatment; for others, it is
used in conjunction with external beam radiation therapy to
the pelvis. Based on PSA screening data in 1991 to 1993, it
was estimated that up to 10% of all newly diagnosed prostate
cancer patients would be considered ideal candidates for seed
implantation as definitive radiotherapy management.1
The techniques for permanent interstitial prostate brachytherapy evolved in two distinct eras. Historically, seed implantation was performed by free-hand placement of seeds in
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an open surgical procedure via the retropubic approach.2 Dosimetric planning was limited to the use of nomographs following intraoperative measurement of the size of the prostate
gland.3–5 The total activity to be implanted was determined
using the average dimensions of the prostate. Postimplant
dosimetry was analyzed in terms of the matched peripheral
dose 共MPD兲, defined as the isodose surface that would cover
a spatial volume numerically equal to the volume of the
prostate inferred from the ellipsoidal approximation.6 Overall, the open surgical technique suffered from substantial uncertainties in dosimetric planning, implant execution and
dose evaluation. In contrast, contemporary techniques for
prostate brachytherapy rely on three-dimensional 共3D兲
image-based treatment planning and real-time visualization
of needle insertion and/or seed deposition. Seed implantation
is performed under template guidance via a transperineal approach in a percutaneous procedure typically performed in
an outpatient surgical setting. Holm et al.7 first described the
use of transrectal ultrasound 共TRUS兲 for precise guidance of
transperineal seed insertion in 1983. The technique was further popularized by Blasko, Grimm, Ragde and
co-workers,8–10 and has evolved into the most popular modality for prostate seed implantation to date. Characteristic of
the technique is the use of TRUS for preoperative dosimetric
planning and intraoperative visualization of needle placement. A somewhat different technique, developed by Wallner et al.,11,12 uses computerized tomography 共CT兲 to identify the target volume for treatment planning; intraoperative
needle placement is verified under fluoroscopy using the urethra as the primary landmark. Compared to the open surgical
technique, these contemporary techniques place considerable
emphasis on 3D conformal dosimetric planning and precise
placement of the planned seed configuration in the patient.
Greater emphasis is also placed on careful patient selection
based on serum PSA levels and Gleason scores.
The clinical experience associated with the retropubic
technique has been a subject of active investigation.13–21 The
clinical results of long term studies with 10–15 year
follow-up have been mixed, partly because the techniques of
seed implantation and hence the implant qualities were quite
varied. In particular, Zelefsky and Whitmore20 recently reported the final assessment of the 15-year outcome of the
historical series of retropubic freehand implants performed at
the Memorial Sloan-Kettering Cancer Center. They concluded that the technique was associated with a greater than
expected incidence of local relapse at 15 years, and identified
suboptimal dose distribution due to technical limitations as
the possible cause of the unfavorable outcome. Nath et al.21
examined the 3D dose distribution of 110 prostate implants
performed at Yale–New Haven Hospital in this era. They
identified a number of dosimetric quality indicators to which
statistically significant differences in local recurrence-free
survival could be attributed. Patients in the dosimetrically
favorable group had 10-year survival rates higher by a factor
of up to 2 compared to those in the unfavorable group. Review of the contemporary transperineal experience using
template and image guidance is still ongoing.22–33 While
some early studies22,31 indicate significant proportions of
Medical Physics, Vol. 26, No. 10, October 1999
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tumor-positive biopsies postimplantation and/or distant metastases, most of the large published series24–26,29 have
shown PSA-based control rates comparable to prostatectomy
or external beam radiation. A notable study by Vijverberg
et al.33 examined biopsy findings postimplantation and the
quality of the implant in terms of the minimum dose delivered to the prostate. They reported significant correlation between the implant quality and the resulting negative biopsy,
and between the implant quality and the serum PSA during
follow-up. One of the major advantages of seed implantation
has been the lower morbidity rates compared to radical prostatectomy and external beam radiation therapy. The use of
contemporary techniques has further reduced treatmentrelated morbidities. Urinary or rectal complication and
sexual dysfunction are generally reported to be relative low
in many recent studies.34–39 Careful treatment planning and
execution are expected to further reduce treatment-related
morbidities.
In summary, there is now considerable evidence to suggest that technical innovations, 3D image-based planning,
template guidance, computerized dosimetry analysis and improved quality assurance practice have converged in synergy
in modern prostate brachytherapy, which promise to lead to
increased tumor control and decreased toxicity. A substantial
part of the medical physicist’s contribution to this multidisciplinary modality has a direct impact on the factors that
may singly or jointly determine the treatment outcome. It is
therefore of paramount importance for the medical physics
community to establish a uniform standard of practice for
prostate brachytherapy physics, so that the therapeutic potential of the modality can be maximally and consistently realized in the wider healthcare community.
Prostate seed implantation is the permanent placement of
radioactive seeds in the prostate using interstitial brachytherapy techniques. However, it differs from traditional
brachytherapy in three important aspects: 3D anatomy-based
dosimetric planning, real-time diagnostic imaging guidance
and fast dose fall-off due to lower energy radionuclides. In
addition, it differs from remote-controlled high dose rate
brachytherapy in that the radioactive source strength distribution is less amenable to optimization and alteration. These
considerations lead to the unique nature of prostate seed implant, which may be characterized as precision-oriented yet
dosimetrically sensitive. To the majority of brachytherapy
practitioners, image-guided interstitial implantation is a relatively new treatment technique. A typical implant team consists of the radiation oncologist, the medical physicist, the
urologist and/or the ultrasound radiologist. At present, most
of the practitioners acquire technical proficiency through a
short training course followed by actual patient treatment.
There is as yet no uniform requirement either in the training
curriculum or within the medical physics profession regarding adequate understanding of the unique physics issues in
seed implantation. An extensive survey by Prete et al.40 in
the U.S. for prostate brachytherapy revealed alarming variance in the pattern of practice in physics and dosimetry, particularly in regard to dose calculation, seed assay and time/
method of postimplant imaging. Because of the large number
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nent prostate seed implant. It should be emphasized that this
is a rapidly evolving treatment modality and an area of active
investigation. Much of our current knowledge in optimized
treatment planning, intraoperative uncertainties, the time
course of prostate volume change, correlation of radiological
studies of the prostate, and postimplantation analysis is based
on research efforts which are still ongoing. The intention of
this report is therefore to guide the practicing medical physicist in successfully implementing or improving the prostate
implant procedure, and to provide a survey of the current
standard of practice in this evolving field.
The remaining parts of this document are organized as
follows: Sec. II provides a practical review of the current
techniques in ultrasound-guided seed implantation; Sec. III
reviews the dosimetric aspects of prostate brachytherapy,
with an emphasis on the present knowledge in treatment
planning, dose specification and reporting; Sec. IV contains
the summary of recommendations; Sec. V discusses issues
for future consideration.
II. REVIEW OF CURRENT TECHNIQUES
A. Overview of contemporary techniques
FIG. 1. Process flow diagram for a preoperatively planned prostate seed
implant.
of start-up programs at this time, it is essential that the roles
and responsibilities of the medical physicist be clearly defined, consistent with the pivotal nature of the clinical physics component in assuring the ultimate success of prostate
brachytherapy.
It was against this background that the Radiation Therapy
Committee of the American Association of Physicists in
Medicine formed Task Group No. 64, which was charged 共1兲
to review the current techniques in prostate seed implant
brachytherapy, 共2兲 to summarize the present knowledge in
treatment planning, dose specification and reporting, 共3兲 to
recommend practical guidelines for the clinical medical
physicist, and 共4兲 to identify issues for future investigation.
Although high dose rate 共HDR兲 brachytherapy for prostate
cancer has certain similarities with permanent seed implantation, the topic is beyond the scope of this Task Group. This
report represents the work of the AAPM Task Group No. 64.
The report has been approved by the Radiation Therapy
Committee and the Science Council.
The dosimetry formalism for interstitial brachytherapy
was standardized by the AAPM Task Group No. 43.41 A
code of practice for brachytherapy physics in general was
outlined by Task Group No. 56.42 The present report will
address the clinical medical physics issues unique to permaMedical Physics, Vol. 26, No. 10, October 1999
The major goal of prostate brachytherapy is to deliver a
tumoricidal dose to the cancer-bearing prostate while minimizing urinary and rectal morbidities. The specific aims are
to design the optimal treatment plan using 3D anatomical
information, to implement the treatment plan intraoperatively
with precision, and to analyze the dosimetric outcome
postimplantation. Contemporary prostate brachytherapy is a
multi-disciplinary treatment modality, in which each member
of the implantation team brings specialized knowledge that
promotes the clinical goal. Figure 1 delineates the flow of
events pertinent to the medical physicist in this treatment
modality. The role of the medical physicist spans the entire
process of patient treatment, from the planning volume
study, dosimetric planning, seed preparation, to intraoperative consultation and radiation safety supervision, and
postimplant dosimetry.
B. Treatment planning techniques
Computerized treatment planning plays an important role
in modern prostate brachytherapy. Careful dosimetric planning leads to smooth and expedient implantation, and reduces the likelihood or extent of normal tissue radiation
damage. The process of dosimetric planning is especially
helpful to the implant team in the early stages of implementing the prostate brachytherapy program. It allows the practitioners to contemplate the technical and dosimetric issues
presented by each case and make adjustment, if necessary,
prior to implantation. Although it is time consuming, the
planning process requires the team to give prior consideration to the patient’s anatomy and any technical problems it
may present. It allows the team to formulate a plan that will:
共1兲 provide coverage of the entire target volume by the prescribed dose while keeping the rectal and urethral doses
within acceptable tolerances, 共2兲 control dose inhomogene-
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ity, and 共3兲 keep the implant as technically simple as possible. The following topics are important in planning the implant:
1. Ultrasound volume study
The ultrasound volume study used to plan the implant is
usually obtained no earlier than 2–3 weeks before implantation, in order to limit changes in the prostate, particularly if
the patient is under hormonal therapy. If it is not possible to
comply with this time interval 共e.g., due to a shortage of
seeds兲, a second volume study and computerized treatment
plan may be performed before implantation. The volume
study consists of consecutive axial images obtained at 5 mm
intervals from the base of prostate to the apex, with the template hole pattern superimposed on each image. Practitioners
who enlarge the planning target volume 共PTV兲 beyond the
prostate often start 5 mm superior to the base and end 5 mm
inferior to the apex. In either case, a sagittal ultrasound image is often obtained for base-apex length measurement to
assure that the proper number of slices are obtained. A member of the physics staff is usually present to ascertain that the
patient is set up in such a manner that a satisfactory plan can
be developed from the volume study. Specific parameters
that are checked include: 共1兲 the angle of elevation of the
patient’s legs in the stirrups; 共2兲 the alignment of the ultrasound probe with respect to the prostate in all of the ultrasound images, such that implant needles, which are inserted
parallel to the probe, do not traverse the rectal wall; 共3兲 the
superposition of the template hole pattern on the contours of
the prostate. In particular, the most posterior aspects of the
prostate need to be within or very close to the posterior row
of template holes in order to adequately cover the prostate by
the prescribed dose.
The planning volume study ideally includes adequate localization of the prostatic urethra on each axial slice. The
seed configuration is then designed to avoid implantation at
or near the location of the prostatic urethra.
2. Pubic arch
The first consideration in the planning process is to determine the degree of pubic arch interference. The pubic arch
may ‘‘shadow’’ the anterior and lateral portions of the prostate, making it difficult or impossible to implant seeds in
these locations. If this restriction exists, the brachytherapist
may angle the template and ultrasound probe assembly to
achieve better needle access. However, the ability to correct
this problem is limited. Severe pubic arch interference is
considered a contraindication for performing the implant.
Both CT and TRUS have been used to detect pubic arch
interference. In the CT-based technique, the largest extent of
the prostate is manually projected onto the axial slice containing the pubic arch. If significant overlap exists between
the two structures, pubic arch interference is likely to be
encountered. A shortcoming of this technique is that the patient is not in the lithotomy position during the CT study,
thus the relationship between the prostate and the pubic arch
may be slightly different during implantation. In the TRUSMedical Physics, Vol. 26, No. 10, October 1999
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based technique, the ultrasound probe is first moved to an
axial slice on which the pubic bone-soft tissue interface is
visible. The location of the pubic arch is traced on the ultrasound screen using the cursor. The probe is then moved longitudinally to visualize the entire prostate on successive axial
slices, with the tracing of the pubic arch overlaid on the
images. The advantage of this technique is that it can be
combined with the volume study, with the patient in the
treatment position. However, it is harder to precisely identify
the pubic arch on ultrasound compared with CT.
3. Seed distribution
Different types of seed distributions are in current use and
a consensus on the optimal seed distribution does not exist.
The classic approach is to space the seeds 1 cm apart, centerto-center, throughout the prostate. This approach, referred to
as uniform loading, requires a higher number of lower
strength seeds 共typically 0.4 to 0.5 U seed for 125I, 1.2 to 1.5
U seed for 103Pd兲, and is characterized by relatively high
doses in the center of the prostate. In modified peripheral
loading, some seeds in the central portion of a uniformly
loaded implant are deleted to reduce the central dose. This
may require increasing the strength of the remaining seeds or
decreasing the needle to needle or seed to seed spacing in the
periphery. Peripheral loading is an alternative approach in
which the seeds are preferentially limited to the periphery of
the prostate. This requires a substantial increase in seed
strength 共typically 0.75 to 1.0 U/seed for 125I, 2.0 U/seed or
higher for 103Pd兲. The end result is to produce a dose minimum 共albeit above the prescribed minimum dose兲, instead of
a dose maximum, at the location of the urethra.
4. Urethra
The prostatic urethra is readily visualized on TRUS and
CT studies when a Foley catheter is left indwelling during
imaging. Alternatively, aerated gel injected into the urethra
can act as a contrast-enhancing agent under TRUS imaging.
In order to plan the treatment to avoid direct implantation
near the urethra or to calculate the dose received by the urethra, the entire length of the prostatic urethra needs to be
visualized. Based on the study by Wallner et al.,38 it appears
that the maximum urethral dose and the length of the urethra
that receives greater than 360 Gy 共converted from 400 Gy of
pre-TG43 dose for 125I兲 are significantly correlated with
RTOG grade 2–3 urinary morbidity. In another study, Desai
et al.43 reported that acute urinary morbidity in 117 patients
treated with 125I implants correlated with the dose-volume
histogram of the prostate as well as doses delivered to 5 cm2
of the urethra as measured by the dose-surface histogram.
Additional studies incorporating more patients, different radionuclides and various seed loading patterns will aid the
determination of the dose tolerance to the urethra.
Treatment plans are commonly devised to limit the urethral dose whenever possible. To accomplish this goal, seeds
are not placed in close proximity to the urethra. In addition,
some seeds in an otherwise uniformly loaded implant may
have to be deleted to achieve this goal.
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5. Rectum
The anterior wall of the rectum is adjacent to the prostate,
which makes it difficult to deliver the prescribed dose to the
posterior periphery of the prostate without delivering an
equivalent dose to the most anterior portion of the rectum.
Placing the seeds too close to the rectal wall may increase
the risk of ulceration, whereas the extreme posterior portion
of the prostate may be underdosed if the seeds are placed too
far away. Particular attention is given to the recto-prostatic
interface in planning the implant. The physicist aims to cover
the entire prostate while keeping the volume of the rectal
wall that receives the prescribed dose as small as possible.
Special care is taken where seeds must be positioned near the
recto-prostatic interface, especially if peripheral loading is
employed using higher activity seeds.
According to a dosimetric study by Wallner et al.based on
CT scans taken 2–4 h after implantation,38 the rectal surface
that receives greater than 90 Gy 共Converted from 100 Gy of
pre-TG43 dose for 125I兲 appears to correlate significantly
with rectal bleeding or ulceration. This study suggests that
either the dose-surface histogram or the amount of the rectal
wall that receives greater than 90 Gy is a useful parameter in
dosimetric planning and analysis. Again, further studies incorporating more patients, different radionuclides and different seed loading patterns will aid the determination of the
rectal dose tolerance.
6. Dose margin
Due to seed placement uncertainties that are inherent to
the implant procedure, the percentage of the prostate volume
that is covered by the prescribed dose is almost always less
than planned. Thus, if the prescribed dose and coverage are
to be achieved it may be necessary to ‘‘over plan’’ the implant. This is achieved in a variety of ways: by using a planning volume that is larger than the prostate volume 共which is
also justified by the known incidence of extracapsular extension of disease兲, by increasing the total activity implanted by
about 15%, or by increasing the number of seeds or seed
strength until the prescribed isodose line lies several millimeters outside the prostate. All of these methods effectively
constitute a planning integral dose escalation. Thus, the decision to plan a dose margin is tied to the prescribed dose
itself.
7. Intraoperative planning
Although preoperative dosimetric planning has been a
community standard in modern prostate brachytherapy, this
two-step process from the planning volume study to implantation contains several sources of uncertainties. The patient
position in the planning volume study is difficult to reproduce in the operating room, leading to ad hoc modifications
of the treatment plan at the time of implantation. Anesthesia
may result in relaxation of pelvic musculature and consequent change in prostate shape compared to the contour obtained in the volume study without anesthesia. Furthermore,
the prostate may undergo volume change in the interval between planning and implantation, which invalidates the preMedical Physics, Vol. 26, No. 10, October 1999
FIG. 2. Process flow diagram for an intraoperatively planned prostate seed
implant.
operative plan. Appreciable change in prostatic volume or
position can occur following the insertion of stabilizing
needles, introducing yet another source of error in implementing the preoperative plan.
These problems can be alleviated by the technique of intraoperative optimized treatment planning.44 With the patient
anesthetized and in the treatment position, the prostate is first
stabilized using implantation needles. A complete TRUS
volume study follows, from which images are transferred to
the planning computer and segmented. The treatment planTABLE I. Equipment requirement for the prostate seed implant program.
Mick applicator technique
Capital
Well-type ionization chamber
GM or scintillation detector
Ion chamber survey meter
Computer treatment planning
system
Ultrasound unit
Stabilization device/attachment
Fluoroscopy unit
Mick applicator
Pre-loaded needle technique
equipment
Well-type ionization chamber
GM or scintillation detector
Ion chamber survey meter
Computer treatment planning system
Ultrasound unit
Stabilization device/attachment
Fluoroscopy unit
Supplies and consumables
Loading block, cartridges
Needle box, 共optional兲 needle
loading device
Seed carrier
Seed sterilization container
Mick-compatible needles
Needles
共Optional兲 stabilization needles
共Optional兲 stabilization needles
Reverse action tweezers
Reverse action tweezers
Radioactive seeds
Radioactive seeds
Spacers and bone wax
2060
Yu et al.: Task Group No. 64
ning system is invoked to produce optimized dosimetric
plan, which is subsequently reviewed and approved by the
physicist and the radiation oncologist in the operating room
before seed placement takes place.
Intraoperative computerized planning allows several major steps to be streamlined. Specifically, preoperative planning can be reduced to a screening of pubic arch interference
and a volumetric measurement to estimate the maximum total source strength required. Seed inventory can be consolidated to the same batch for a group of patients, which reduces the variance for waste. Figure 2 shows the modified
flow of events pertinent to the medical physicist under the
intraoperative technique. Most notably, intraoperative dosimetric planning is a critical step in this treatment technique,
where the medical physicist occupies an important role in the
intense decision-making process.
C. Equipment and applicators
Equipment for ultrasound-guided prostate implants includes the ultrasound machine, the rectal probe, the stepping
device/probe carrier, the perineal template, and the stabilizing mechanism 共see Table I兲. The ultrasound machine is
typically a portable unit, and contains a seed implant software package such that a grid pattern can be displayed on the
screen. The stepping device allows the rectal probe to be
attached to the stabilizing mechanism while permitting
movement in and out of the patient’s rectum in precise steps.
The needle template has holes accepting 17 gauge or 18
gauge needles, arranged typically in a 13 by 13 matrix, at 5
mm spacing. The template may be designed to mount directly to the rectal probe in some commercial systems, in
which case it moves together with the probe, or it may be
mounted on the probe carrier, in which case it remains stationary with respect to the perineum as the probe is moved.
In either case, the holes on the needle template correspond to
the grid points displayed on the TRUS monitor screen. The
stabilizing mechanism immobilizes the entire rectal probe/
carrier/template system against the operating table or floor,
to prevent unintentional motion of the probe and needle template during the implant procedure. The template is placed at
close proximity to the perineum to minimize needle splaying
in the target volume.
Ultrasound equipment is now available that can display
the sagittal as well as transverse planes of the prostate volume. This feature has been found to be helpful in identifying
the superior prostate capsule to guide individual needle insertion, in visualizing the movement of the prostate volume
as needles are inserted, and in confirming that the seeds are
deposited correctly at the cephalad-most portion of the
prostate.45–47
Equipment required for the prostate volumetric study includes ‘‘stirrups’’ to support the legs and an examination
table that allows the mounting of stirrups. The patient is set
up in an extended lithotomy position on the examination
table, with the thighs at approximately right angles to the
body. This rotation of the pelvic bones allows better access
to the prostate, and helps avoid needle obstruction by the
Medical Physics, Vol. 26, No. 10, October 1999
2060
pubic arch. It is important that similar stirrups are available
to achieve the same patient position during the implantation
procedure in the operating room, so that the relationship of
the prostate and the adjacent organs can be accurately and
quickly reproduced. The patient is prepared for the volumetric study using Fleets enema, and may be catheterized using
a Foley catheter with saline injected into the bladder and the
Foley bulb in order to identify the urethra. Alternatively,
aerated gel may be injected as a contrast agent to identify the
urethra by ultrasound.
A lubricant 共such as K-Y jelly™兲 is needed to help introduce the rectal probe into the rectum. Topical anesthesia
共such as Lidocaine jelly™兲 may be needed for sensitive patients. A small amount of saline or coupling jelly should be
injected into the condom over the rectal probe for improved
imaging quality and positioning of the posterior prostate capsule in the ultrasound image. Too much saline in the condom
may however cause distortion of the prostate and the neighboring anatomy. The amounts of saline injected into the
bladder, the Foley bulb, and the condom may be recorded, to
be reproduced in the implant procedure.
D. Source type, assay and preparation
125
I and 103Pd sources are comparable in photon energy,
capsule dimensions and dose distribution. 125I and 103Pd are
encapsulated in titanium and delivered as sealed sources
共‘‘seeds’’兲. They are similar in size 共4.5 mm⫻0.8 mm outer
dimensions for 125I model 6711 seeds and 4.5 mm
⫻0.81 mm for 103Pd model 200 seeds兲. Both 125I and 103Pd
decay via electron capture. 125I and 103Pd are currently produced by nuclear reactors and cyclotrons, respectively.
The 125I decay scheme results in the emission of photons
with energies of 27.4 keV 共1.15 photons/disintegration兲, 31.4
keV 共0.25/dis兲 and 35.5 keV 共0.067/dis兲.48 At the time of this
report, the seed type most commonly used for prostate implants is model 6711, which contains 125I in the form of
silver iodide deposited on the surface of a silver rod. This
silver rod also serves as a radiographic marker. 125I model
6711 seeds therefore also emit fluorescent x-rays resulting
from photoelectric interaction in the silver rod, the energies
of which are 22.1 keV 共0.15/dis兲 and 25.5 keV 共0.04/dis兲.
The average energy for all emissions is approximately 27.4
keV, which results in a half value layer in lead of approximately 0.025 mm. The self absorption of this assembly is
approximately 37.5%. Therefore, contained activity is approximately 1.6 times the apparent activity. The air kerma
strength used for prostate implants is commonly between 0.4
and 1.0 U 共0.3–0.8 mCi兲 per seed. The halflife of 125I is 59.4
days; ninety percent of the total dose is delivered in 197
days.
103
Pd emits characteristic x-rays of 20.1 keV 共0.656/dis兲
and 23.0 keV 共0.125/dis兲.48 The half value layer in lead is
0.008 mm. The active radionuclide is plated onto two graphite pellets on either side of a lead radiographic marker within
the titanium capsule. Each end of the seed is cupped inward
共i.e., it is concave兲. This is a salient feature of the 103Pd seeds
and therefore can be used to uniquely identify 103Pd seeds.
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Yu et al.: Task Group No. 64
共Other seeds are convex where the two end welds meet the
main source body.兲 The self-absorption of the seed is stated
to be 54% by the manufacturer, indicating that the contained
activity of 103Pd in each seed is approximately 2.2 times the
stated apparent activity. The air kerma strength commonly
used for prostate implants is between 1.4 to 2.2 U 共1.1 to 1.7
mCi兲 per seed. The half-life of 103Pd is 16.97 days; 90% of
the total dose is delivered in 56 days.
AAPM Task Group No. 56 recommends that 10% of the
seeds be assayed.42 There are several seed assay methods
that address the special circumstance in which a large number of loose seeds are contained in a shipment. Seeds can be
assayed in bulk, or in cartridges.49 In addition, an autoradiograph can be taken with a large number of seeds to compare
the resulting film density. This film method, in conjunction
with the well chamber seed assay, assures that the seeds are
of uniform strength. Seeds in suture are delivered sterile,
thereby complicating the assay procedure. A calibration
check may be performed on single nonsterile seeds from the
same batch as a given shipment of seeds in suture and ordered expressly for this purpose. Alternately, a sterile insert
to the standard dose calibrator can be used to directly assay
seeds in suture, as described by Feygelman et al.50 and by
Butler et al.51 The advantage of the latter techniques is that
sterility may be maintained while a sufficient number of
seeds are assayed.
Unlike seeds in suture, loose seeds are not sterile and
need to be sterilized prior to use. The method of sterilization
depends on the implantation technique. If preloaded needles
are used, the seeds are sterilized prior to loading. After sterilization, the needles are loaded under sterile conditions, often in the operating room. If the Mick applicator is used, the
seeds can be loaded into cartridges prior to sterilization. The
sterilized cartridges are then taken to the operating room,
ready for use.
Seeds are commonly sterilized in an autoclave. Flash sterilization can be used, or longer duration steam sterilization
may be opted if time and availability allow. Flash sterilization is done in the autoclave at 270 °F 共133 °C兲 at 30 PSI for
at least 3 minutes. The conventional autoclave cycle is
250 °F 共121 °C兲 at 15 PSI for about 30 minutes. Loose seeds
can be sterilized in the vial/lead pig in which they are delivered, with the cap loosened. Alternatively, the vial can be
uncapped and the open end plugged with cotton. The gravity
cycle is preferable to the vacuum cycle when loose seeds are
sterilized. The difference between gravity and vacuum cycles
is the drying method. The vacuum cycle uses a strong
vacuum to achieve drying. The vacuum may displace loose
seeds from the container, causing potential radiation hazard.
Seeds may also be sterilized using ethylene oxide gas 共cold
gas兲. Cold gas sterilization takes considerably more time and
is required for seeds in suture material.
Seed preparation requirements depend on the implantation
technique adopted, i.e., using the Mick applicator or preloaded needles. Table I shows a list of typical equipment
required for either technique. The needles used for implantation also differ. The Mick TP200 applicator compatible
needles have a blunt needle sheath and a stylet that is slightly
Medical Physics, Vol. 26, No. 10, October 1999
2061
longer than the sheath, with a trocar point. Needles used in
the pre-loaded technique have a sharp beveled point, and a
blunt stylet that is slightly shorter than the needle. A mark is
typically present on the end of the needle to indicate the
direction of the bevel at the tip. This can be helpful in that
the needle track may be made to deflect slightly toward the
beveled direction, if so desired. The needles and stylets also
have centimeter markings to help visually determine the
depth of needle insertion, and the length of the needle that is
filled with seeds and spacers. The first 0.5–1 cm of the
needle is usually sand-blasted for increased ultrasonic
echogenicity. If the image of the needle tip on ultrasound is
used to infer the actual needle depth, it is important to assess
the precise depth where the needle first appears under ultrasonic imaging; otherwise a systematic positioning error of
0.5–1 cm can occur.
In the Mick applicator technique, 125I or 103Pd seeds are
loaded into the Mick-compatible cartridges using the loading
block and a pair of reverse action tweezers. The loaded cartridges are then screwed into either the loading block or the
seed carrier and sterilized. Some seeds are commercially
available in pre-loaded plastic cartridge inserts that are compatible with the Mick applicator. Use of such pre-loaded inserts minimizes radiation exposure to the personnel and the
time required for loading the cartridges. The user’s well
chamber calibration factors may, in this case, be obtained
specifically for such pre-loaded cartridge inserts.
The pre-loaded needle technique requires longer time for
preparation. In this case, the radioactive seeds and spacers
are loaded into sterilized needles as specified by the treatment plan. The loading pattern of seeds vs. spacers for each
needle may be printed on a diagram to facilitate the loading
process. The needle tip is plugged with a piece of surgical
bone wax or rectal suppository. The length of the wax 共approximately 5 mm兲 should be accounted for when depositing
the seeds into the prostate. The loaded needles are then
placed into a sterilized needle box, ready for implantation.
Various needle loading devices are commercially available
that aim to reduce the amount of time required for loading
the needles, permit visual verification of the loading pattern,
and reduce radiation exposure to personnel.
A Geiger–Muller 共GM兲 counter or scintillation detector is
used to survey the seed preparation area after completion of
the loading process. A running total of the seeds is kept as
they are loaded into cartridges or needles.
In early 1995, Amersham Healthcare introduced Rapid
Strand™ in which the I-125 seeds are enclosed within a stiff,
absorbable suture material that maintains the seeds 1 cm
apart center-to-center. The suture material is braided Vicryl™ 共polyglactin 910兲 which is stiffened thermally and sterilized by ethylene oxide. The stiffened Vicryl suture material
is hygroscopic and softens and swells when exposed to moisture from body fluids. If not handled properly, the strand
may swell and jam in the implant needle making it impossible to expel the strand from the needle. Therefore, it is
important that the needle be plugged properly. Bone wax,
which is often used to seal the tip of needles loaded with
loose seeds and spacers, is too hard to expel without causing
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Yu et al.: Task Group No. 64
a mechanical collapse of the Vicryl spacing between the
seeds. Suppositories, such as Anusol-HC™, are a softer material that partially melts at 37 °C and allows expulsion of the
strand without collapsing the Vicryl. The internal bore of the
needle must be very smooth. Any roughness of needle wall
will catch the Vicryl fibers and jam the needle.
The technique for use of Rapid Strand can be summarized
as follows.52 Place 3 suppositories in a 30 ml glass cup or
beaker and flash sterilize in a steam autoclave for 3 min at
140 °C and 2 atm pressure. With aseptic technique, an empty
18 gauge implant needle is dipped vertically into the clear,
molten suppository to a depth at least covering the needle
bevel and preferably covering the burnished echogenic region of the needle 共approximately 5 mm兲. Upon removing
the needle vertically, capillarity and gravity equalize to form
a liquid column 7–9 mm long that solidifies in 3–10 min.
Once the needles have cooled for 5 min, appropriate lengths
of Rapid Strand are cut and inserted into the needles, followed by the stylet. After inserting the needle into the patient, the moisture resistance of the suppository seal persists
even though the needle may be retracted and reinserted several times in achieving the desired location. During this time
the suppository is warming and melting. Approximately 2–3
min at body temperature are required for the suppository to
melt sufficiently around the perimeter of the needle so that
the strand can be easily expelled. Two circumstances that
may lead to jamming of the strands within the needle are
leaving too little time for the suppository to melt sufficiently,
or striking the pubic bone, which may dislodge or disrupt the
suppository plug.
E. Implantation procedure
Ultrasound-guided prostate implant procedures can be
performed in an operating room or an interventional radiology procedure room. After anesthesia, the patient is set up in
the dorsolithotomy position, and draped with sterile covers.
The perineum is cleaned with Betadine™ and the scrotum is
retracted by either sewing it to the drapes or by using a sling.
The ultrasound probe is attached to the stepping device and
inserted into the rectum. With the exception of the needle
template, these instruments are usually nonsterile, but are
cleaned prior to use.
The prostate may be first immobilized against lateral and
anterior-posterior motion by use of two or more stabilizing
needles, inserted under TRUS visualization through carefully
chosen template positions.53,54 Special prostate stabilizing
needles with a hook-type mechanism are commercially available, though standard implant needles for stabilization are
also effective.53 In general, three stabilizing needles arranged
in a triangular pattern inside the prostate are quite adequate.
However, not all brachytherapy practitioners use immobilization needles.
Under the current preoperatively planned implantation
technique, the positioning of the prostate on the template
grid under TRUS is carefully checked against the treatment
plan both before and after the insertion of the stabilizing
needles. Offset in the relative position by exactly one grid
Medical Physics, Vol. 26, No. 10, October 1999
2062
width 共5 mm兲 is easily corrected by shifting the treatment
plan. Other offset amounts need to be remedied by repositioning the patient. Under the intraoperative planning technique, prostate stabilization is followed by TRUS volume
study and optimized dosimetric planning, as described in
Sec. II B.
For either the Mick applicator technique or the pre-loaded
needle technique, a needle loading diagram and/or worksheet
is required in the operating room to identify the template
coordinates for needle insertion. In the Mick applicator technique, the loading diagram specifies the spacing between the
seeds in each needle, the number of seeds in the needle, and
the distance of the first seed-drop position from the base of
the prostate 共or any other reference plane兲, i.e., the offset
from the reference plane. The diagram for the pre-loaded
needle technique specifies the offset from the reference plane
for each needle.
The operating table ideally allows the placement of a mobile fluoroscopy unit to visualize the implanted area. The use
of fluoroscopy during implantation helps in visualizing the
needles and seeds as they are inserted in relation to a Foley
bulb filled with contrast media, and the patient’s bony
anatomy.55 As a needle is inserted into or pulled out of the
prostate, the movement of the previously implanted seeds
can be readily seen on the fluoroscopy monitor, and adjustment in the needle insertion depth may be made to compensate for such movement.
If needles are to be loaded in the operating room, a sterile
table or work area equipped with adequate radiation shielding is set up by a member of the physics staff. Even in the
Mick applicator technique, a set of sterile loading and seed
handling equipment is kept available in case jammed cartridges need to be reloaded.
During seed placement, the medical physicist interprets
the planning information in each incremental step to the clinicians, and records the progress of seed deposition in the
patient. Typically, the medical physicist provides verbal instructions on the needle coordinates, the offset from the reference plane, and, if using the Mick applicator, the number
of seeds and the seed spacing, as each needle is being placed.
The technique for seed placement requires some degree of
manual dexterity. In the pre-loaded needle method, the
needle is withdrawn against the stylet such that the seeds
remain in the same position in the prostate as the needle is
removed. Advancing the stylet will deposit the seeds ahead
of their intended locations, while allowing the stylet to retract with the needle will cause the seeds to be deposited
behind their intended locations. The Mick applicator method
requires the clinician to keep track of each seed deposition.
Desired seed spacing is achieved by retracting the needle a
known number of steps on the applicator’s preset scale.
When the needle is retracted too rapidly, the seed tends to
follow it due to the suction created by needle retraction.
When the stylet is advanced too rapidly, the seed can be
injected beyond its intended location in the prostate. Care is
taken to ensure that only one seed is allowed to drop from
the cartridge at a time, otherwise multiple seeds will be
placed at one planned location.
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Yu et al.: Task Group No. 64
It is often found that preoperative treatment plans require
minor or major modifications. Typical scenarios that cause
such modifications include discrepancies between expected
and actual needle placement, detection of the urethra or adjacent rectal wall close to the intended seed location, prostate
volume/position change, or the need to compensate for prior
seed misplacement. The medical physicist brings to the operative procedure a special understanding of the dosimetric
impact of any such modification, and thus plays an important
role in evaluating each circumstance. It should be stressed
that while real-time deviation from the preoperative plan is
often unavoidable for adequate dose coverage, excessive ad
hoc deviation can lead to severely suboptimal dose distribution. To acquire a quantitative understanding of the dosimetric characteristics of a preoperative plan, the medical physicist may wish to simulate a number of such ad hoc changes
on the planning computer prior to implantation. This is especially helpful in the early stages of a prostate seed implant
program.
A running total of the seeds and needles implanted is
recorded on the operative worksheet. Any real-time deviation from the planned seed deposition is annotated as it occurs. In the Mick applicator technique, it is good practice to
reconfirm the running total of seeds deposited every time the
cartridge is emptied, which serves as a checksum for correct
seed drop. This is to be facilitated by loading a constant
number of seeds per cartridge 共except the last cartridge兲.
Note that the checksum method does not guard against accidental placement of multiple seeds at the same location, but
will uncover the problem soon after it occurs. A radiation
survey should be made for each used needle to confirm that
no seed is unintentionally left inside the needle.
After seed placement, the urologist usually performs a
cystoscopy to find and retrieve any loose seeds in the bladder. A lead seed container is kept available in the operating
room, for use in the event that seeds are retrieved from the
bladder, or that seeds are accidentally dropped on the floor.
A GM detector or a scintillation detector is kept available to
locate misplaced seeds, and to conduct radiation survey in
the implantation area following the procedure. The radiation
survey includes the floor, waste, linen and all used applicators. All seeds brought to the operating room must be accounted for by the implantation worksheet and the seeds that
remain in the possession of the medical physicist at the end
of the procedure. A properly calibrated ion chamber survey
meter is used to measure the maximum exposure rate at the
surface and at 1 m from the implanted patient for documentation.
F. Postimplant dosimetry
1. Rationale
The quality of prostate seed implants is, as in all brachytherapy, dependent upon the skill and experience of the practitioner. Because patients differ in their anatomy, some implants are technically more difficult than others. Hence, a
variation in implant quality may occur, even for an experienced practitioner.
Medical Physics, Vol. 26, No. 10, October 1999
2063
Prostate implants are generally planned to deliver a prescribed minimum dose. However, it has been shown that the
minimum dose planned can rarely be achieved due to seed
placement errors which are inherent in the procedure.56–58
Furthermore, postimplant edema can further reduce the dose
delivered by the implant.59–61 Hence it cannot be assumed
that the patient would receive the dose prescribed in the pretreatment dosimetric plan.
Postimplant dosimetric evaluation was traditionally carried out using multiple radiographs. Although such plane
films are adequate for reconstruction of the relative seed positions, they cannot provide the dose delivered to the prostate
because the prostate cannot be visualized on a radiograph.
Postimplant dosimetry was limited to a calculation of the
matched peripheral dose 共MPD兲, a parameter that has been
shown to be an unreliable indicator of the dose delivered to
the prostate.62
The dose delivered to the prostate and other organs can be
determined by performing a postimplant CT-based dosimetric analysis. The advantage of CT-based dosimetry is that the
prostate and other organs, such as the rectum, can be visualized. This capability allows dose-volume histograms 共DVHs兲
to be generated, which provide detailed information on dose
coverage and implant quality.
At present, a postimplant CT study is the most direct
method for carrying out quantitative dosimetric evaluation.
CT-based dosimetric evaluation is particularly important
during the early stages of a new prostate seed implant program to aid the team in progressing up the learning curve as
quickly as possible. Continuous evaluation of implant quality
permits improvement in techniques as the program develops.
Otherwise, problems which compromise implant quality may
go undetected and be perpetuated indefinitely.
2. Technical issues
The necessary steps in performing a CT-based dose analysis are 共1兲 outlining the prostate volume for dosimetric evaluation on each CT image, 共2兲 localization of each seed, 共3兲
calculation of the dose to each point in a 3D matrix of grid
points in a selected volume which includes the prostate, 共4兲
generation of isodose curves which can be superposed on
each CT image, and 共5兲 generation of a DVH for the prostate
as well as dosimetric information for the critical structures.
A seed 4.5 mm in length often appears on adjacent CT
images spaced at 5 mm intervals, therefore a useful facility
in dosimetric analysis is to permit identification of seeds that
appear on multiple adjacent CT images. This is usually accomplished by superposing the seed location from the previous image onto the image being analyzed.62 Seed redundancy algorithms are also helpful, which can reduce the
seeds to the number actually implanted using distance-based
redundancy likelihood analysis.
A complete CT-based dosimetric evaluation includes the
dose delivered to other organs, such as the urethra and rectum. However, there are no standards for specifying the dose
to these organs, and each case presents a unique set of circumstances. It is very difficult, if not impossible, to define
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Yu et al.: Task Group No. 64
the urethra on a CT image unless there is a Foley catheter in
the urethra. Distension of the rectum can cause variability in
assessing the rectal dose due to the typical large dose gradient in this region.
The determination of the dose to the prostate from a
postimplant CT scan is nontrivial. A major problem is defining the prostate volume accurately on the CT images. Outlining the prostate on CT involves subjective judgment because the prostate is not well resolved from other adjacent
soft tissue structures. As a result, the volume derived from
the CT scan is generally larger than that of the TRUS volume
study used to plan the implant.63–65 The problem this presents is that the dose coverage will be evaluated for a prostate volume which is larger than that used in planning the
implant. As a result, the percentage of the ‘‘prostate’’ covered by the prescribed dose will generally be less than that
planned.
The most notable difficulties in defining the prostate in
CT images have been described63 as 共1兲 an inability to distinguish the posterior portion of the prostate from the anterior wall of the rectum on noncontrast CT. 共2兲 a tendency to
confuse the posterior-inferior 共apical兲 portion of the prostate
with the anterior portion of the levator ani muscles, and 共3兲 a
tendency to include portions of the neurovascular bundles as
part of the prostate volume. Because of these difficulties,
defining the prostate requires a certain amount of subjective
judgment.63
Another problem is postoperative edema, which typically
increases the prostate volume by 40 to 50% compared to the
preoperative volume.59,60,66 If the postimplant CT scan is obtained immediately after the implant is performed, the dose
may be underestimated. The edema increases the distance
between the seeds, as well as the volume, thereby lowering
the dose rate. On the other hand, if the CT study is obtained
after the edema has resolved, the dose may be overestimated
because the decrease in dose rate while the prostate was
edematous is ignored.
The impact of edema on the postimplant dosimetry is not
yet well defined. However, two factors which intuitively
contribute are: 共1兲 the magnitude of the edema and 共2兲 the
margin used in planning the implant. An example of a margin is an implant planned so that the prescribed isodose line
is a few millimeters outside the periphery of the prostate.
Such margins are created by the practice of increasing the
planned source strength by approximately 15% to compensate for seed placement error.63 One would expect a greater
percentage of the edematous prostate to be covered by the
prescribed isodose line when such a margin is incorporated
into the plan.
The scheduling of postimplant imaging studies is an important quality assurance issue because of the effect of
edema on the postimplant dosimetry. However, the optimal
time for obtaining the CT scan has not been established. The
optimal time for imaging 125I and 103Pd implants will differ
because their half-lives are different. The duration of edema
is a key factor in determining the optimal timing. A recent
study60 based on serial CT scans shows that the edema resolves exponentially with a half-life of from 4 to 25 days
Medical Physics, Vol. 26, No. 10, October 1999
2064
共mean: 9.3 days兲. Using the mean edema half-life of 9.3
days, the edema will typically resolve to 12.5% of its original value in 28 days. This would appear to be an appropriate
time to image an 125I implant because of its 60 day half-life.
However, the situation is not so clear with 103Pd because of
its much shorter 17 day half-life.
Although the methodology has not yet been perfected, the
TRUS volume study may ultimately become a useful aid in
defining the prostate volume in the CT study.65 This is particularly true if the postimplant CT scan is obtained after the
edema is resolved so that the preimplant and postimplant
volumes can be assumed to be equivalent. If both studies
were imaged at 5 mm intervals, the TRUS study should be
useful in identifying the apex in the CT study even before a
methodology for registering TRUS and CT images becomes
available. Fusion of CT and MR images may also be a viable
solution, as MR provides adequate visualization of the prostate while CT provides localization of the implanted
seeds.66,67
These numerous difficulties and technical challenges notwithstanding, the standards for seed implant quality are being defined in terms of quantitative CT-based dosimetric
evaluation. Willins and Wallner68 reported that, for CT scans
obtained on the day of implantation, coverage of 80% or
more of the target volume by the prescription dose is probably adequate. Bice et al.69,70 have conducted extensive review of postimplant dosimetry using a wide range of CTbased quality assessment parameters. Stock et al.71 found
that dose was the most significant predictor of biochemical
failure in a multivariate analysis using dose, PSA, Gleason
score and stage in 134 patients treated with 125I implants. A
dose response was observed at a level of 140 Gy in D90, the
dose that covers 90% of the target volume under CT-based
postimplant evaluation. Patients receiving a D90 less than
140 Gy had a 4-year freedom from biochemical failure rate
of 68%, compared to a rate of 92% for patients receiving a
D90 greater or equal to 140 Gy (p⫽0.02).
The clinical correlation of dosimetric evaluators is an ongoing effort. Practitioners of prostate brachytherapy are
urged to carefully document the methodology and the time
course for each set of postimplant dosimetry, in order to
preserve its predictive value.
III. REVIEW OF DOSIMETRIC ASPECTS
This section reviews the dosimetric aspects of treatment
planning and postimplant analysis relevant to permanent
prostate brachytherapy. The historical circumstances that led
to the adoption of the ‘‘160 Gy prescription dose’’ are described, so that contemporary practitioners can make an intelligent judgment with regard to the past clinical experience
based on the Memorial nomograph. Dosimetric consistency
with the AAPM Task Group No. 43 formalism is again
stressed. Methods for dosimetric evaluation and optimization
are summarized.
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Yu et al.: Task Group No. 64
A. Historical perspective on dosimetry
Although 222Rn seeds, 198Au seeds and even 192Ir seeds
have been used in permanent implants of the prostate, the
historical background of clinical and physics techniques pertinent to the present report really began with the initial use of
125
I seeds for this purpose at New York’s Memorial Hospital
in the late 1960’s.72 These implants were performed using a
retropubic approach, following a midline incision and bilateral lymphadenectomy. Ideally, needles were inserted about
1 cm apart and parallel to one another, avoiding the urethra
and stopping short of the rectum by sensing needle pressure
on a finger in the rectum. Each needle was withdrawn at least
0.5 cm before the first seed was inserted. The dimensions of
the prostate were assessed in the plane perpendicular to the
needles as the distance between peripheral needles and along
the needle direction by subtracting the average needle protrusion from the overall needle length 共15 cm兲. The total
apparent activity 共in mCi兲 to be implanted was determined
by multiplying the average dimension 共in cm兲 by an empirically derived factor of 5. Implementation of this procedure
was eventually facilitated by a nomograph that specified the
number of seeds 共of known strength兲 to be implanted and
their approximate spacing within the target.3
The dose associated with an implanted activity determined in the above manner was believed to be about 160 Gy
and was considered to be the minimum effective dose. However, an early evaluation of the ‘‘dimension averaging’’
method had shown that, on the basis of Quimby volume
implant data,73 for which the cumulated activity 共mg h兲 per
unit dose is approximately proportional to the square root of
the volume treated, dimension averaging may be expected to
produce a peripheral dose roughly proportional to the minus
one-sixth power of volume.5 Alternately, if Manchester volume implant data74 had been invoked, where the cumulated
activity per unit dose is explicitly stated to be proportional to
the two-thirds power of volume, the expectation would have
involved a dose proportional to the minus one-third power of
volume. In either of these conjectures, the premise is that 125I
seeds display a dose-rate fall-off with distance from the seed
similar to that from a radium or radon source. 共Dimension
averaging had, in fact, been used earlier for radon seed implants.兲 Although we now know that assumption to be totally
unjustified, it is nevertheless clear that adherence to the
original dimension-averaging method of planning leads to
smaller doses for larger target volumes. During the time this
planning method was in use, it was not really appropriate to
suggest that it resulted in delivery of a given dose, since
target volumes 共e.g., prostates兲 varied significantly in size.
Before the advent of CT imaging, there was no way to
evaluate the minimum peripheral dose received by the prostate, since the prostate capsule was not seen on the postimplant stereo-shift radiographs from which dose calculations
were generally performed. In order to provide some form of
feedback to the brachytherapy clinicians that would reflect
the extent to which implant goals were achieved, it became
customary at Memorial Hospital to report the dose for which
the isodose contour volume was the same as the target volMedical Physics, Vol. 26, No. 10, October 1999
2065
TABLE II. Average dose at total decay from a source with an air kerma
strength of 1 U using the point source approximation.
Dose at total decay 共Gy兲
Distance
共cm兲
125
I
model 6711
1985 NIST
Standard
125
I
model 6711
1999 NIST
Standard
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
70.02
16.83
6.93
3.50
1.97
1.18
0.74
0.49
0.33
0.23
0.17
0.12
0.09
0.07
77.98
18.74
7.71
3.90
2.19
1.32
0.83
0.54
0.37
0.26
0.19
0.14
0.10
0.08
103
Pd
model 200
TheraSeed®
20.19
3.91
1.33
0.56
0.27
0.13
0.07
0.04
0.02
0.01
ume inferred from dimensions measured at surgery.75 The
target was usually approximated as an ellipsoid, for which
the volume is the product of the three dimensions multiplied
by ␲/6. Because it involved matching volumes, the dose so
reported subsequently came to be called the ‘‘matched peripheral dose’’ or MPD. It was obtained by interpolation in a
table of volumes computed at uniformly spaced dose levels,
each volume representing the sum of voxels for which the
dose was greater than the specified dose.
The MPD concept proved helpful in later modifications of
the original ‘‘planning’’ nomograph to take into account the
difference in dose rate falloff with distance between 125I
seeds and 222Rn seeds.6,76 MPD values evaluated for actual
implants were found to decrease significantly with increasing
target volume.77 From the slope of the line fitted to such data
on a log–log plot, it was possible to derive the exponent that,
if applied to the average dimension in a modified dimensionaveraging method, would result in a constant dose as a function of target volume. This exponent was found to be 2.2,
where the increase over the 2.0 value that would have been
implied by Manchester volume implant data was taken to be
due to the much lower penetration of 125I photons relative to
those of 222Rn. The same reasoning was later applied to develop a similar nomograph for 103Pd seed implants.4 For
103
Pd, with photons of even lower energy, the corresponding
exponent of average dimension was 2.56. For both 125I and
103
Pd nomographs, the ‘‘constant-dose’’ formula was applied
only for average dimensions greater than 3 cm. For smaller
implants, the original dimension-averaging rule was allowed
to stand and the dose increased for decreasing volume.
With respect to the ephemeral but pervasive ‘‘160 Gy’’
prescription dose for 125I permanent implants, a further observation of interest is that it seems to have survived in spite
of major changes in 125I dosimetry. At the time it was first
proposed, dose calculations were using a one-dimensional
lookup table of dose rate times distance-squared with an en-
2066
Yu et al.: Task Group No. 64
2066
try at a distance of 1 cm of 1.7 cGy cm2 mCi⫺1 h⫺1. This
value had been arrived at indirectly, making use of TLD
measurements of relative dose vs distance in Mix-D phantom
material. It was considered to be the quotient of total energy
emission per mCi h and the product of phantom density and
the volume integral of the relative measurements.78 It was
changed, in 1978, to 1.10 cGy cm2 mCi⫺1 h⫺1 on the basis of
subsequent measurements and calculations79 that included
averaging the anisotropy 共albeit in air兲 over 4␲ solid angle.
MPD values were reduced to 65% of what they would have
been using the previous table.
B. Dosimetry data revision
The history of 125I dosimetry has been reviewed in detail
by both the AAPM Task Group 43 共TG43兲 report41 and the
report of the ad hoc Committee of the AAPM Radiation
Therapy Committee on 125I sealed source dosimetry.80 As
indicated by Kubo et al.,80 two separate but related events
need to be considered in the discussion of dosimetry data
revision for 125I seeds:
共1兲 The adoption of the AAPM Task Group No. 43 recommended dosimetry data for model 6711 125I seeds, which
differs significantly from the dosimetry data of Ling
et al.81
共2兲 The revision of the NIST calibration standard for the
titanium-encapsulated 125I seeds, including both model
6702 and model 6711 seeds. This revision causes the
reported air kerma strength value for a calibrated seed to
be approximately 10% lower than that under the current
NIST calibration standard.
When applied to the dosimetry of permanent prostate seed
implants, the isotropic point source approximation is commonly used for the dose calculation model. At distance r
from the center of the source, the dose delivered at total
decay from an 125I or 103Pd seed is
D⫽Ḋ 0 ⫻1.443⫻T 1/2 ,
共1兲
where the initial dose rate Ḋ 0 is given by the TG43 formalism
Ḋ 0 ⫽
¯ an
S k ⌳g 共 r 兲 ␾
,
2
r
共2兲
¯ an are the air kerma strength, dose
where S k , ⌳, g(r), and ␾
rate constant, radial dose function, and anisotropy constant,
respectively. Table II gives the dose at total decay from an
125
I or 103Pd seed with an air kerma strength of 1 U, using the
point source approximation. Comparison with the old exposure rate formalism shows that, for a model 6711 125I seed of
given strength, the dose calculated using the TG43 formalism and dosimetry data is approximately 11% lower at 1 cm
from the center of the seed in water. Luse et al.82 further
compared the isodose distribution for the implant of a 35 cc
prostate, using 88 seeds and 20 needles. They found that the
contiguous volume of 53 cc receiving 160 Gy, using the Ling
et al. dosimetry data, would receive 144 Gy when the dose is
Medical Physics, Vol. 26, No. 10, October 1999
calculated using the TG43 dosimetry data, corresponding to
the 11% difference observed earlier. Luse et al. therefore
recommends that the prescription dose in prostate implants
using 125I seeds be modified downward by 11%, prescribing
144 Gy instead of 160 Gy, when the dosimetry data of TG43
is used in the dose calculations. Another empirical study by
Bice et al.83 based on similar comparative analysis also
reached the same conclusion.
While there is no timeline constraint on the adoption of
the TG43 dosimetry data, early adoption of TG43 will facilitate smooth implementation of the revised air kerma strength
standard by NIST. The revision of the NIST I-125 standard
will need to be carried out in concert with the manufacturer.
This requires a change of the dose rate constant ⌳ in the
user’s treatment planning system. Loevinger84 has reported
earlier that this revision would lead to approximately a 10%
increase of the dose rate constant. The AAPM Radiation
Therapy Committee Ad Hoc Subcommittee on Low-Energy
Seed Dosimetry reviewed the available data and
recommends85 that, for I-125 sources calibrated using the
1999 NIST air-kerma-strength standard, the dose rate constants should be revised upward by a factor of 1.114. In
particular, it is recommended that dose rate constants of 0.98
cGy/h-U and 1.04 cGy/h-U be used for model 6711 and
model 6702 seeds calibrated using the NIST 1999 revised
air-kerma-strength standard.
C. Dose specification and reporting
Consistency in dose specification, prescription and reporting is an important step towards establishing a uniform standard of practice in prostatic brachytherapy. Early efforts in
this area86–90 were exclusively limited to idealized representations of the target using cubic, cylindrical, spherical or ellipsoidal volumes. However, these investigations marked the
departure from built-in target-size dependence in
nomograph-based planning and prescription toward specification or prescription of a desired dose. The early experience
with CT-based planning and evaluation for 125I prostate implants at the Memorial Sloan Kettering Cancer Center was
reported by Roy et al.62 In their study, the peripheral dose
was defined as the isodose surface that encompassed 99% of
the target volume, or D99. By analyzing 10 implant cases,
they showed that the actual coverage of the target volume by
the peripheral dose ranged from 78% to 96%, with an average coverage of 89%. More recently, Willins and Wallner68
published a follow-up study presenting the analysis of 20
unselected implant cases performed by an experienced clinical team. In this study, dose was prescribed to the planned
minimum peripheral dose 共mPD兲; postimplant dosimetry was
carried out using CT taken on the day of implantation. The
actual coverage of the target volume by the prescribed mPD
ranged from 73% to 92%, with an average of 83%. The
actual D100 delivered to the target volume was 43%⫾8%
共⫾1 SD兲 of the prescribed mPD. The authors identified subjectivity in interpreting postimplant CT scans and prostatic
swelling as extraneous uncertainties, which would compound
with any seed placement errors to result in apparently poor
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Yu et al.: Task Group No. 64
coverage by the mPD. The reliance on the mPD for dose
prescription and reporting was discussed by Yu et al.56
Based on simulated distributions of common seed placement
errors, they concluded that generally 90% of the original
PTV could be covered by the prescribed mPD when the average dimension of the PTV was greater than 3 cm. No consistent pattern was found for the magnitude of underdosage
between the planned mPD and the realized D100; however,
underdosage from the planned mPD to the realized D100
could easily exceed 20% due to common seed displacement.
Although the mPD as a dose specification parameter displays excessive sensitivity, it is the most direct measure of
dosimetric coverage under 3D image-based treatment planning. Less sensitive dosimetric parameters have been proposed for prescription and/or reporting, including the net
minimum dose,89 the average peripheral dose,88 the harmonic
mean dose,90 and the widely cited matched peripheral dose.
However, these parameters by definition do not provide the
essential information on isodose coverage of the PTV. The
practice of prescribing the treatment dose to the mPD is also
consistent with modern external beam radiation therapy. The
problem of achieving consistency between dose specification
for prescription and dose reporting is closely related to the
uncertainties in defining the PTV, which are discussed in
more detail in Sec. III D.
It must be concluded at this time that the present techniques for permanent seed implantation do not yet allow the
planned mPD to be reproduced with any consistency; however, the percent coverage of the PTV by the planned mPD
isodose surface is a reasonably consistent indicator of implant quality. Furthermore, the mPD required for clinical
control of disease and avoidance of morbidity is currently
unknown.
With regard to regulatory compliance, this body of research work strongly suggests that apparent underdosage
from the prescribed mPD to the realized D100 for a given
implant cannot be taken alone as evidence of poor administration of brachytherapy.
D. Dosimetric uncertainties
Three major sources of uncertainties are now widely recognized: seed displacement, prostate edema postimplantation, and difficulty in defining the target volume based on
CT. These uncertainties can potentially compound to cause
gross dosimetric variability and ultimately to affect the treatment outcome.
Seed displacement refers to the deviation in the positions
of the implanted seeds from the planned locations. The dosimetric impact of seed displacement has been well
documented.56,57,68,91 In particular, Roberson et al.57 classified seed displacement in terms of needle placement error,
source-to-source spacing variability, and seed splaying.
These errors arise because 共1兲 the patient position during the
planning volume study is not always reproducible in the operating room; 共2兲 the prostate volume may have changed
since the planning study, particularly for patients under hormonal therapy; 共3兲 prostate movement occurs during implanMedical Physics, Vol. 26, No. 10, October 1999
2067
tation, even with stabilizing needles in place. Use of rigidly
spaced source strands will partially alleviate the seed displacement uncertainties. However, a complete solution to the
above problems is unlikely to result until intraoperative computerized real-time dosimetry is widely available.
Dosimetric uncertainties as a result of prostate edema and
difficulty in defining the target volume based on CT are
unique to dose analysis and reporting postimplantation.
These issues are discussed in detail in Sec. II F.
E. Treatment plan evaluation
Methods for dosimetric evaluation are necessary in order
to select competing treatment plans during the traditional
planning process, in ranking computer-optimized plans, or in
postimplant dosimetry analysis. At present, little clinical correlation has been published between the planning dosimetry
evaluators and treatment outcome, due in part to the difficulty of achieving the planned parameters in actual implants.
To that extent, the dosimetric evaluators are pragmatic constructs to quantify the treatment plan evaluation process.
1. S mPD
The total source strength required to deliver 1 Gy of the
mPD, S mPD , is implicity or explicitly used in treatment planning to select the seed distribution that yields the maximum
tumor dose. Intuitively, the dose distribution within a given
PTV is also most uniform when S mPD is minimized, for otherwise some source strength can be removed from the nonuniform region without reducing the mPD. The concept of
S mPD can be traced back to the Manchester system of implant
dosimetry. For prostate seed implants using 125I and 103Pd
seeds, the following fitted results provide minimized values
of S mPD as functions of the average dimension d of the
PTV:56,92
125
I
103
Pd
S⫽0.014 d 2.05 U/Gy-mPD,
S⫽0.056 d 2.22 U/Gy-mPD.
共3兲
共4兲
These relationships do not take into account any irregularities in the shape of the target volume, and therefore should
only be used as idealized estimates of S mPD . Most clinical
treatment plans are likely to yield higher values of S mPD . For
the purpose of benchmarking comparison with the idealized
model, the three largest orthogonal dimensions of the isodose
surface selected for prescription may be used to obtain the
average dimension.
For reference, the following fitted results are obtainable
from Ref. 4 for the total source strength implanted per Gy of
MPD:
125
I
S⫽0.012 d 2.2 U/Gy-MPD
共converted to TG43 dose兲.
103
Pd
S⫽0.36 d 2.56 U/Gy-MPD.
共5兲
共6兲
For average dimensions d between 3 and 5 cm, Eqs. 共3兲–共4兲
and Eqs. 共5兲–共6兲 agree to within 10% for 125I or 103Pd. It is
well known that the mPD is substantially lower than the
MPD. However, Eqs. 共5兲–共6兲 were derived from the dosim-
2068
Yu et al.: Task Group No. 64
etric analysis of past clinical cases, i.e., arising from postoperative analysis, whereas Eqs. 共3兲–共4兲 were results of optimized ideal seed placement, i.e., arising from preoperative
planning. The apparent agreement of the two sets of fitted
equations is therefore rather incidental.
2. Dose uniformity
The full-width at half-maximum 共FWHM兲 of the differential dose-volume histogram or the ‘‘natural’’ dose-volume
histogram93 has been used as an indicator of dose uniformity
for prostate implants.62,94 Greater dose uniformity throughout
the target volume would be associated with a reduced
FWHM. Roy et al.62 reported 307 Gy⫾73 Gy 共⫾1 SD兲 in
FWHM of the DVH postimplantation for 10 cases in 1993.
Although they did not report the FWHM in the corresponding preoperative plans, the parameter is likely to increase
from the plan to the postimplant dosimetry because seed displacement tends to spread out the peak of the DVH. A simpler construct is the uniformity number 共UN兲, defined as the
ratio of the mean peripheral dose to the mean tumor dose,
both calculated as harmonic means in the PTV to avoid numerical instability.90 The UN is about 0.7 for idealized implants, and should be relatively insensitive to seed displacement. Dose profiles have also been used to measure
uniformity in two dimensions through the target volume.95
The notion of achieving dose uniformity is related to the
concept of sterilizing a uniform distribution of tumor cells
throughout the PTV, which is also an implicit assumption in
most external beam treatment of prostate cancers. With the
possible exception of minor dose heterogeneity,96 high doses
within the PTV in excess of that required to produce sufficient cell kill is assumed to add risk without benefit to
therapy.
3. Dose conformity
Dose conformity measures the closeness between the isodose surface chosen for prescription and the PTV in three
dimensions. It is different from dose uniformity, as demonstrated in the following example. If the PTV is a sphere, then
a point source located in the center will achieve perfect conformity, but the dose distribution is severely nonuniform
throughout the target. Thus dose conformity alone does not
fully define the objectives for optimizing prostate implant
treatment plans.
The most common measure of dose conformity is the
root-mean-square deviation of the peripheral dose from a selected dose level. This evaluator is often used in conjunction
with computerized optimization. Another conformity indicator is the peripheral uniformity number 共PUN兲, defined as the
ratio of the mPD to the mean peripheral dose calculated in
the harmonic formalism.90 For planned seed configurations
optimized for 125I and 103Pd, the PUN is on average equal to
0.67. A higher PUN is indicative of better conformity in the
treatment plan. The PUN is likely to undergo severe degradation after seed displacement due to the volatility in the
mPD. A third parameter proposed in the literature is the conformation number 共CN兲, which is applicable to both external
Medical Physics, Vol. 26, No. 10, October 1999
2068
beam and brachytherapy.97 When calculated at the level of
the mPD, the CN is simply the ratio of the volume of the
PTV to the volume enclosed by the mPD isodose surface.
For prostate seed implants, the CN is on average 0.72, compared to 0.65 for a 3-field external beam boost treatment of
the prostate.
The notion of dose conformity is based on the assumptions that a well-defined and clinically relevant PTV can be
precisely identified, and that there is reasonable expectation
of delivering the conformal dose distribution as planned. In
some institutional protocols, the PTV includes an expanded
dosimetric margin around the true prostate to encompass extracapsular extension and in anticipation of dose degeneration subsequent to seed misplacement. Treatment planning
then aims to conform to the expanded PTV. However, distortion of the planned isodose surface invariably occurs due
to seed placement uncertainties. Until a technique becomes
available that substantially accounts for these uncertainties in
real time, dose conformity remains an evaluator only of idealized treatment plans.
4. Dose-volume histogram
Compared to single scalar evaluators, the DVH 共including
‘‘natural’’ DVH兲 provides substantially more information for
quantitative evaluation of the dose distribution associated
with a given plan or actual implant. Variations of the DVH
concept include the coverage, external-volume and heterogeneity indices,21,56,98 and the dose nonuniformity ratio.98 In
particular, the coverage index 共CI兲 shows the percentage of
the target volume covered by any isodose level. In the preoperative treatment plan, CI at the mPD dose level is by
definition 100%; in the actual implant, the corresponding CI
at the same dose level should be approximately 90% under
the current implantation techniques, and is expected to be
higher with better techniques.56
F. Treatment plan optimization
It was recognized from the early days of image-based 3D
planning that template-guided prostate implants were amenable to computerized optimization. Roy et al. first reported
the Memorial experience of CT-based optimized planning, in
which the seed loading patterns were determined by a leastsquare method to maximize the dose conformity;99 the
needle patterns were selected on the basis of clinical judgment. This method is therefore semi-automatic, in that the
treatment planner needs to design a needle pattern 共and in
their initial approach, needle orientation兲 as a starting point
for computer optimization. Even so, the authors reported a
factor of 10 reduction in the planning time compared to the
manual planning experience. The optimal seed loading rules
were explored by Narayana et al.100 When the effects of potential seed displacement and prostatic volume change were
taken into account, peripheral loading appeared to be the
optimal strategy.
A number of robust optimization schemes have since been
applied to prostate implants. Pouliot et al.101 used simulated
annealing 共SA兲 to optimize a cost function that took into
2069
Yu et al.: Task Group No. 64
account both dose conformity and dose uniformity. The genetic algorithm 共GA兲 has been adapted for inverse planning
to minimize S mPD and maximize dose conformity, while
keeping the number of needles at a customary range.92,102
Chen et al.103 devised an ad hoc method in which one seed
was placed at a time until S mPD was minimized. Overall,
these optimization techniques were able to produce improved
treatment plans, as measured by the respective evaluators,
without extensive human intervention and within 1–15 min
run time on modern computers. In addition, both SA and GA
are stochastic, ‘‘intelligent’’ optimization schemes, capable
of search for optimality as defined by realistic objective
functions.104,105 The potential for incorporating seed displacement uncertainties in such an intelligent optimization
scheme was explored.102,106 It is quite likely that some of
these techniques will be translated into mainstream treatment
planning and intraoperative dosimetric guidance in the near
future.
IV. CURRENT RECOMMENDATIONS
Based on the rationale and the empirical evidence outlined in the foregoing sections, the AAPM recommends the
following practical guidelines as a basis for promoting a
level of quality standards in prostate brachytherapy physics
that is necessary to ensure that the anticipated clinical outcomes are reproducible and uniform on a large scale. Practitioners of existing prostate seed implant programs are urged
to compare these guidelines with their institutional protocols,
carefully evaluate and justify any departures, and make
modifications to their programs if necessary. For new prostate seed implant programs, it is recommended that these
guidelines be implemented as part of the quality assurance
protocols.
It is recognized that modern prostate brachytherapy is a
multi-disciplinary effort that involves radiation oncology, diagnostic radiology and urology. Successful implementation
and continued improvement of a prostate brachytherapy program rely on effective teamwork and ongoing quality assurance review of the entire program. The physics aspects of
quality assurance as outlined in this section are an integral
part of this multi-disciplinary effort.
This section of the document uses three distinct levels of
imperatives with strictly defined meanings:
共1兲 Shall or Must indicates a recommendation that is necessary to ensure a minimum standard of safety and effectiveness in prostate seed implantation;
共2兲 Should indicates a recommendation that is necessary to
meet the baseline standard of practice in prostate seed
implantation;
共3兲 Recommend indicates an advisory recommendation that
is to be applied when practicable.
A. Equipment
The medical physicist should be directly involved in the
selection, acceptance-testing and quality assurance of any
equipment acquired for the prostate seed implant program
Medical Physics, Vol. 26, No. 10, October 1999
2069
共see Table I兲. The quality assurance of equipment that affects
the dosimetric consequences of seed implantation must be
performed by the medical physicist.
Imaging: Verification shall be made 共e.g., in phantom兲 to
ensure that the grid pattern on the ultrasound image corresponds to the physical locations given by the perineal template. The fluoroscopy unit used in the operating room
should display minimal distortion in a screen area that adequately encompasses the implant region. It is recommended
to identify and follow a set of acceptance testing and ongoing quality assurance procedures described in the Report of
AAPM Ultrasound Task Group No. 1107 that are relevant to
TRUS imaging, especially with regard to spatial resolution,
grayscale contrast, geometric accuracy, and distance measurement.
Accessories: Proper functioning of applicators, accessories and stabilizing devices should be verified before each
implantation procedure.
Treatment planning system: The medical physicist shall
verify that the treatment planning system reproduces the values shown in Table II for the dose at total decay from an 125I
model 6711 or 103Pd seed, calculated using the TG43 data in
the point source approximation.40 This test serves as a necessary indication that the planning system complies with the
dosimetric formalism recommended by TG43. The medical
physicist shall verify that the treatment planning system performs the correct dose summation at one or more locations in
a simple configuration of multiple seeds. We endorse the
recommendations of the AAPM Task Group No. 40108 regarding quality assurance of treatment planning systems. In
particular, the above tests shall be performed before the computer treatment planning system is put into clinical use, and
at each subsequent software release.
Dosimeters: The medical physicist shall establish the calibration of dosimeters for the assay of each type of seed used
in the prostate brachytherapy program. The user’s well
chamber shall be calibrated at an ADCL with direct traceability to NIST. Alternatively, individually calibrated seeds
shall be obtained from the ADCL to establish a calibration
factor for the particular geometry being used. In either case,
the constancy of the user’s dosimeter shall be confirmed using a long-lived radionuclide before each use. Proper functioning of the ion chamber survey meter and radiation detector shall be verified using a long-lived test source before
each use in the operating room.
B. New radionuclide designs
New supplies of 125I, 103Pd or other low energy sources
for permanent implantation need to become commercially
available to meet the increasing demand for radioactive
seeds. However, it must be stressed that for any new therapeutic radionuclide, the dosimetry for the source design must
be established. Ideally, a national air kerma strength standard
should be established, and the parameters for the TG43 formalism should be measured and independently confirmed.
The dosimetry of low energy photon-emitting brachytherapy sources such as 125I and 103Pd is sensitive to the
2070
Yu et al.: Task Group No. 64
source geometry, encapsulation and internal structure due to
self-absorption effects. These factors can be particularly sensitive to the quality of the manufacturing process during seed
fabrication. It is inappropriate to use the dose rate constants,
radial dose functions, anisotropy functions, anisotropy factors or constants published in the TG43 report for new
source designs. Regarding a source intended for wide use,
the vendor shall have the responsibility to provide a calibration of source strength that is traceable to a standard, and the
medical physicist shall have the responsibility to ensure that
the clinical dosimetry parameters have been validated by independent investigators other than the vendor.
C. Seed assay
Radioactive seeds may be obtainable in loose seeds,
ready-loaded cartridges, or absorbable suture. In whatever
form the seeds are procured, the manufacturer’s assay must
be independently confirmed. As recommended by AAPM
Task Group No. 56,41 a random sample of at least 10% of the
seeds in the shipment should be checked. Discrepancies of
3% or more between the mean of the assay and the manufacturer’s calibration should be investigated. Unresolved discrepancies of 5% or more should be reported to the manufacturer.
As discussed by the Ad Hoc Committee of the AAPM
Radiation Therapy Committee on 125I sealed source dosimetry, the revision of the NIST standard for 125I must be taken
into account as soon as it becomes available.
D. Changes to the dose value due to TG43
To promote uniformity in the clinical adoption of the
TG43 formalism, it is recommended to scale the prescribed
dose such that a pre-TG43 value of 160 Gy becomes 145 Gy.
This recommendation is based on the discussion in Sec. III B
but with the dose rounded from 144 Gy to 145 Gy. The
clinically optimal dose and the method of prescription are
not yet definitive. In cases where a pre-TG43 prescribed dose
other than 160 Gy needs to be converted to the TG43 value,
it is recommended to use the scaling ratio of 0.9.
E. Dosimetric planning
Treatment planning must be carried out for all patients
prior to the insertion of radioactive seeds. In this context,
treatment planning refers equivalently to intraoperative planning or conventional preoperative planning. It is recommended to generate the isodose distributions superposed on
the contours of the prostate in selected planes, and to construct the DVH for the prostate. It is recommended to generate the DVH or ideally the dose-surface histogram 共DSH兲 for
the rectum. It is recommended to adequately identify the entire length of the prostatic urethra, and to calculate the dose
profile along the urethra.
Prior to implantation, the dosimetric plan should be
checked using an independent procedure or by a second
member of the physics staff, and must be reviewed by the
radiation oncologist.
Medical Physics, Vol. 26, No. 10, October 1999
2070
F. Implantation procedure
A member of the physics staff shall be present in the
operating room during prostate seed implantation. The physics personnel must be familiar with the treatment plan and
the dosimetric consequences of any deviation from the plan.
If the implantation technique relies upon preoperative plans
based on prior volume studies, the position of the prostatic
gland relative to the template coordinates must be verified in
more than one imaging plane. If deviation from the planned
position is detected, the physics personnel should evaluate
whether modification to the setup and/or treatment plan is
required, and recommend corrective action.
An account of the needles and seeds implanted shall be
kept as the procedure progresses. At the end of implantation
and after cystoscopy, the physics personnel shall confirm the
total number of seeds implanted in the patient and the number of seeds remaining, which must add up to the total number brought into the operating room.
A scintillation detector or GM counter must be available
in the operating room. For implants using loose seeds, it is
recommended to survey each needle after it is withdrawn
from the patient, to verify that no seed is unintentionally left
in the needle core. At the completion of the procedure, a
complete radiation survey must be conducted, which includes the vicinity of the implant area, the floor, waste, linen
and all used applicators. The exposure rate at the surface and
at 1 m from the patient should be measured by a properly
calibrated ion chamber survey meter, and documented in accordance with pertinent federal and state regulations.
The physics personnel should be familiar with any institutional policies and procedures regarding sterile techniques
and the operative environment.
G. Patient release
The medical or health physicist shall routinely review the
patient survey results postimplantation to confirm that the
prostate seed implant program continually satisfies all pertinent federal and state regulations regarding the release of
patients with radioactive sources. NCRP Commentary No.
11, ‘‘Dose Limits for Individuals Who Receive Exposure
from Radionuclide Therapy Patients,’’ provides additional
information that may be of use in this context.
For obvious reasons, the institution’s accountability of radioactive sources for a permanent prostate seed implant ends
at the time of patient release. However, basic instructions to
the patient on identifying the seeds and on radiation protection principles should be provided. It is not necessary to
require the patient to strain urine and return dislodged seeds.
H. Postimplant analysis
A quantitative dose analysis must be carried out for each
patient postimplantation. This statement is based on the
premise that it is as important to know and document the
dose delivered by a permanent seed implant as by an external
beam treatment. The importance of a postimplant analysis
cannot be overemphasized for the purposes of multi-
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Yu et al.: Task Group No. 64
institutional comparison, improving techniques, evaluating
outcome, and identifying patients who might benefit from
supplemental therapy or be at risk for long-term morbidity.
The postimplant analysis should include two-dimensional
dose distributions on which the target volume for dose evaluation is outlined. In addition, it is recommended to construct
the DVH for this target volume, and to document the dose
levels that cover 100%, 90% and 80% of the target volume
for postimplant evaluation, i.e., D100, D90 and D80, and the
fractional volume receiving 200%, 100%, 90% and 80% of
the prescribed dose, i.e., V200, V100, V90 and V80. Current
literature suggests that imaging studies for dosimetric evaluation are ideally obtained 2–3 weeks postimplantation for
103
Pd implants and approximately 4 weeks postimplantation
for 125I implants. However, it is recognized that logistic considerations sometimes preclude such uniform timing of
postimplant imaging for all patients. In addition, future technology may permit immediate postimplant dosimetry assessment in the operating room. In any case, the time course of
postimplant dosimetric evaluation should be recorded for all
patients.
For dosimetric evaluation performed at the optimum imaging time, it is recommended to use D90, in comparison to
the prescribed dose, as an indicator of implant quality in dose
coverage. An implant with good coverage is characterized by
D90 equal to or greater than the prescribed dose.
It is recognized that such dosimetric analysis is sensitively
dependent upon the definition of the target volume for
postimplant evaluation. Therefore a consistent radiological
interpretation of the target volume should be used and documented to facilitate future interpretation of the dosimetric
outcome.
It is recommended to construct the DVH or ideally the
DSH for the rectal wall. Furthermore, it is recommended to
adequately identify the location of the prostatic urethra, and
to document the dose to the urethra. We recognize that visualization of the urethra at the recommended imaging time,
rather than immediately postimplantation, may involve additional catheterization. It is hoped that a more convenient
contrast-enhancing technique will become available in the
near future.
I. Training requirement for physics personnel
For a member of the physics staff to perform independent
work in permanent prostate brachytherapy, it is recommended that a minimum of five documented cases be performed under the direct supervision of an experienced physicist. In this context, an experienced physicist is one 共a兲 who
satisfies the above training requirement, or 共b兲 who has performed a minimum of 20 documented cases independently.
J. Recommendations regarding commercial treatment
planning systems
It is recommended that the TG43 dose calculation formalism be explicitly represented in commercial treatment planning systems for prostate seed implantation. This implies that
the half-life, the dose rate constant, the radial dose function,
Medical Physics, Vol. 26, No. 10, October 1999
2071
and as a minimum the anisotropy constant are separately
specified in the source data library. Thus if future changes
are to occur on any of the parameters for a given radioactive
source, they can be easily and uniformly updated by the user
of the system with the least confusion.
It is recommended that software facilities be implemented
to generate the DVH for the target volume and the DSH for
the rectal wall, both in preoperative planning and for postimplant evaluation.
In addition to these practical guidelines, the medical
physicist should observe the recommendations of AAPM
Task Group No. 56 with regard to the code of practice for
brachytherapy physics,42 and of AAPM Task Group No. 40
with regard to quality assurance for radiation oncology in
general.108
V. ISSUES FOR FUTURE CONSIDERATION
This section contains a discussion of some dosimetric and
radiobiologic effects on which consensus understanding does
not yet exist among investigators of prostate brachytherapy.
In addition, areas of current and future investigation are identified to aid practising medical physicists in designing their
clinical seed implant programs.
A. Anisotropic dose calculation
The AAPM TG43 report41 contains extensive tabulation
of the anisotropy functions for 125I and 103Pd single seeds. In
principle, it is not difficult to incorporate the anisotropy
function formalism at the planning stage. However, in so
doing one needs to make certain assumptions about the orientation of the radioactive seeds, e.g., being perfectly aligned
along the needle insertion direction. Such an assumption is
probably quite valid in the case of seed strands compared to
loose seeds, but for any given case it is impossible to predict
the extent and direction of splaying that will occur. On the
other hand, the anisotropy constant is an averaged quantity
weighted by the solid angle, and therefore represents the best
estimate of the dose surrounding a radioactive seed of indeterminate orientation. Use of the anisotropy function formalism in postimplant dosimetry is technically more difficult,
since the orientation of each seed must be determined by
locating both ends of the seed. Until automated seed reconstruction software becomes widely available, the point
source approximation appears to be the more appropriate formalism.
The dosimetric effects of anisotropy for 125I were discussed by Ling et al.109 There appear to be rather large differences in the dose distribution and the mPD between the
anisotropy function formalism and the point source approximation widely used at present. To maintain uniform standard
of dose reporting in prostate seed implant, investigators are
urged to document the dose calculation formalism in their
planning and/or postimplant dosimetry procedures.
B. Interseed effect
Mutual attenuation by neighboring seeds has been reported to be significant.110,111 Meigooni et al.111 performed
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Yu et al.: Task Group No. 64
Solid Water measurement and Monte Carlo calculations to
examine the dose perturbation in a two-plane implant of 3
⫻3 seed arrays. They concluded that the inter-seed effect
would reduce the peripheral dose by 6% for 125I seeds. While
the dosimetric impact of the inter-seed effect may be of clinical concern in simple, regular configurations, the overall effect in prostate implants is not clear. In practice, only the
nearest-neighbor seeds are likely to produce appreciable dose
perturbation. The solid angle sustained by a seed at 1 cm
average distance is sufficiently small that the volume of perturbation is for most purposes negligible.
C. Tissue heterogeneity
The major cause of tissue heterogeneity is calcified deposits in the prostate gland, which occur in a small percentage of
patients. The calcification presents on TRUS and CT studies
as hyperechoic and high density regions, in contrast to the
surrounding fibromuscular tissue. In the energy range of 125I
and 103Pd radionuclides, where the photoelectric effect is the
dominant absorption process, the presence of calcium (Z
⫽20) in fibromuscular tissue (Z⫽7.6) leads to three dosimetric effects: 共a兲 the dose rate constant is different in the two
media, resulting in different absorbed dose; 共b兲 the radial
dose function is modified by the increased attenuation of the
high Z material; 共c兲 increased dose deposition occurs in the
soft tissue at the interface of heterogeneity, due to a greater
number of photoelectrons, which have a range of about 10
␮.112–117 The overall dosimetric effect depends on the extent
and the microscopic structure of calcification and the implant
configuration. As a first approximation, the ratio of mass
energy attenuation coefficients of calcium to muscle is 24 at
30 keV, and 23 at 20 keV. At present, there is no clinical
study to gauge the actual impact of such tissue heterogeneity.
It is prudent to identify patients who present with tissue heterogeneity under planning radiological studies, and to evaluate the efficacy and the optimal strategy of seed implantation
on an individual basis.
The same physical principles may lead to variability of
dose deposition in malignant versus normal histologies due
to different elemental compositions. It is not yet clear
whether the physical laws translate to a therapeutic advantage for adenocarcinoma of the prostate, and if so, what the
magnitude of such advantage is.
D. Biological models
The linear-quadratic cell-kill model was extended by Dale
to take into account 共a兲 the decaying dose rate in brachytherapy, 共b兲 dose rate difference across dosimetric inhomogeneity, 共c兲 tumor cell proliferation, and 共d兲 repair of sublethal damage for low dose-rate radiation.118,119 This model
was used by Ling et al. to examine the effect of dose heterogeneity in prostate seed implants.96 The authors concluded
that there might be some advantage in dose heterogeneity at
about 20% above the prescribed dose, but beyond that, dose
would be ‘‘wasted’’ in terms of producing cell kill. The biologically effective dose 共BED兲 and cell surviving fractions
predicted based on the model have been used as endpoints to
Medical Physics, Vol. 26, No. 10, October 1999
2072
compare alternative treatment plans in prostate implant
optimization.92 Assumptions on the following parameters
must be made to apply the Dale model: the ␣-to-␤ ratio, the
value of ␣, the potential doubling time for tumor cells (T pot),
and the mean time for repair of sublethal damage. Given the
uncertainties in these parameters, it must be concluded that
the biological models should not be taken as quantitatively
predictive, but rather as a guide of the relative efficacy of
competing treatment plans. In addition to the cell surviving
fraction, the tumor control probability 共TCP兲 can be calculated based on the BED with additional assumptions on prostate tumor dose response data.96,120
The commonly quoted prescription dose of 115 Gy for
103
Pd implants is the dose estimated to have the same ‘‘timedose-factor’’ 共TDF兲121 as that corresponding to 145 Gy 共converted from 160 Gy of pre-TG43 dose兲 from 125I implants.
Using the linear quadratic model to compare the relative cell
kill effectiveness of the two radionuclides for these doses,
Ling122 has shown that 103Pd may be more effective for T pot
of a few days and that 125I may be more effective for longer
T pot . The determination of the most efficacious dose for each
type of implants involves ongoing analysis of clinical outcome, dosimetric specification and radiobiological modeling.
E. Relative biological effectiveness
The relative biological effectiveness 共RBE兲 was measured
by Ling et al.123 for 125I and 103Pd and by Freeman et al.124
and Marchese et al.125,126 for 125I. Relative to 60Co and at
dose rates relevant to permanent prostate implant, the RBE
was reported to be about 1.4 for 125I and about 1.9 for
103
Pd. 123 The enhanced cell inactivation for a given dose
reflects the additional biological effects of the radiation that
are not described by the physical quantities.
These radiobiological effects are currently not taken into
account in the clinical dosimetry for prostate seed implant,
and are considered theoretical advantages of this modality.
F. Time course of target volume change
Work is continuing on characterizing the time course of
prostatic volume change subsequent to seed implantation. A
more complete understanding of this issue will have a strong
impact on optimal dosimetric planning and postimplant
analysis. Any significant differences in the pattern of gland
swelling and resolution may be dosimetrically compensated
for in planning for the specific radionuclide used, thus reducing the variance in the effective treatment dose delivered
across the patient population.
G. Differential dose planning and delivery
The notion of planning the dose distribution to encompass
the primary foci of the tumor in a high dose region is often
an attractive one in treatment planning. It is justified radiobiologically on the basis that higher dose is required to eradicate higher tumor cell density. Advances in tumor imaging
for prostate cancer will lend more credibility to the concept
of differential interstitial irradiation. It is the nature of
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Yu et al.: Task Group No. 64
brachytherapy to accept certain high dose regions in the
treatment volume in order to achieve reasonably uniform
dose coverage at a lower isodose level. With computerized
optimization strategies, it is possible to routinely achieve this
goal by placing the high dose volume at the focal area of the
gross tumor. If the goal is achieved without sacrificing any
other aspects of the clinical treatment plan, then it may be
hypothesized that differential dose planning offers a therapeutic advantage compared to dosimetric planning without
regard to the locations of tumor foci.
H. Intraoperative seed localization and dosimetry
While postimplant dosimetry is important for quantitative
evaluation of dosimetric outcome, prostate brachytherapy ultimately will benefit from intraoperative seed localization
followed by real-time computerized dosimetry, all performed
with the patient still under anesthesia and in the treatment
position. Thus any significant underdosage can be discovered
and remedied by additional implantation before the end of
the procedure. There is then a reasonable expectation that
every implant will deliver the intended dose, where the only
dosimetric variability is due to ongoing edematous reaction.
I. Correlation of dosimetric and clinical outcomes
A number of studies21,33,38,71 suggest that the dosimetric
outcome of a prostate seed implant ultimately plays a major
role in predicting the likelihood of local relapse and/or longterm treatment-related morbidity. The natural progression of
the disease is such that extensive follow-up is required for
clinical outcome analysis. Such clinical correlation with dosimetric predictors will be aided by more consistent postimplant analysis and quantitative, organ-specific dose evaluation on a larger scale. Careful treatment plan optimization
and dosimetric outcome analysis will in turn provide an early
indication of treatment effectiveness for a prostate seed implant. Such is the goal of the present effort in seeking wide
success of interstitial implantation in the management of
prostate cancer.
ACKNOWLEDGMENTS
We thank W. S. Bice, W. M. Butler, S. Pai, B. R. Prestidge, and R. G. Stock for helpful discussions on the content
of this report and various aspects of the current practice in
prostate brachytherapy. We also thank members of the Radiation Therapy Committee of the AAPM chaired by Jatinder Palta for careful review of the report.
a兲
This report represents the recommendations of the American Association
of Physicists in Medicine 共AAPM兲 for permanent prostate brachytherapy.
b兲
Electronic mail: yu@vlab.medinfo.rochester.edu
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