Recommendations of the American Association of Physicists in Medicine
regarding the Impact of Implementing the 2004 Task Group 43 Report
on Dose Specification for 103Pd and 125I Interstitial Brachytherapy
Jeffrey F. Williamsona兲
Chair, Photon-Emitting Brachytherapy Dosimetry Subcommittee of the Radiation Therapy Committee,
Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298
Wayne Butler
Schiffler Cancer Center, Wheeling Hospital, Wheeling, West Virginia, 26003
Larry A. DeWerd
Accredited Dosimetry and Calibration Laboratory, University of Wisconsin, Madison, Wisconsin, 53706
M. Saiful Huq
Department of Radiation Oncology, University of Pittsburgh Cancer Institute, Pittsburgh, Pennsylvania,
15232
Geoffrey S. Ibbott
Radiological Physics Center, M.D. Anderson Cancer Center, Houston, Texas, 77030
Zuofeng Li
Department of Radiation Oncology, Washington University, St. Louis, Missouri, 63110
Michael G. Mitch
Ionizing Radiation Division, National Institute of Standards and Technology, Gaithersburg, Maryland,
20899
Ravinder Nath
Department of Therapeutic Radiology, Yale University, New Haven, Connecticut, 06510
Mark J. Rivard
Department of Radiation Oncology, Tufts-New England Medical Center, Boston, Massachusetts, 02111
Dorin Todor
Consultant, Photon-Emitting Brachytherapy Dosimetry Subcommittee, Department of Radiation Oncology,
Virginia Commonwealth University, Richmond, Virginia, 23298
共Received 23 November 2004; revised 14 February 2005; accepted for publication 14 February 2005;
published 27 April 2005兲
In March 2004, the recommendations of the American Association of Physicists in Medicine
共AAPM兲 on the interstitial brachytherapy dosimetry using 125I and 103Pd were reported in Medical
Physics 关TG-43 Update: Rivard et al., 31, 633–674 共2004兲兴. These recommendations include some
minor changes in the dose-calculation formalism and a major update of the dosimetry parameters
for eight widely used interstitial brachytherapy sources. A full implementation of these recommendations could result in unintended changes in delivered dose without corresponding revisions in the
prescribed dose. Because most published clinical experience with permanent brachytherapy is based
upon two widely used source models, the 125I Model 6711 and 103Pd Model 200 sources, in this
report we present an analysis of the dosimetric impact of the 2004 TG-43 dosimetry parameters on
the history of dose delivery for these two source models. Our analysis indicates that the currently
recommended prescribed dose of 125 Gy for Model 200 103Pd implants planned using previously
recommended dosimetry parameters 关AAPM 103Pd dose prescription: Williamson et al., Med. Phys.
27, 634–642 共2000兲兴 results in a delivered dose of 120 Gy according to dose calculations based on
the 2004 TG-43 update. Further, delivered doses prior to October 1997 varied from 113 to 119 Gy
for a prescribed dose of 115 Gy compared to 124 Gy estimated by the AAPM 2000 report. For 125I
implants using Model 6711 seeds, there are no significant changes 共less than 2%兲. Practicing
physicians should take these results into account when selecting the clinically appropriate prescribed dose for 103Pd interstitial implant patients following implementation of the 2004 TG-43
update dose-calculation recommendations. The AAPM recommends that the radiation oncology
community review this report and consider whether the currently recommended dose level 共125 Gy兲
needs to be revised. © 2005 American Association of Physicists in Medicine.
关DOI: 10.1118/1.1884925兴
Key words:
1424
103
Pd, 125I, permanent interstitial brachytherapy, air-kerma strength, dose prescriptions
Med. Phys. 32 „5…, May 2005
0094-2405/2005/32„5…/1424/16/$22.50
© 2005 Am. Assoc. Phys. Med.
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Williamson et al.: Dose specification for
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Pd and
125
I interstitial brachytherapy
I. INTRODUCTION
In April 2000,1 the American Association of Physicists in
Medicine 共AAPM兲 published recommended administered-toprescribed dose ratios, 共DTx / DRx兲t, for 103Pd permanent seed
implants. These ratios, which are a function of time period t,
describe the systematic impact of changes in source-strength
standards and single-source dosimetry parameters on clinical
dose specification. The originally published 共DTx / DRx兲t ratios
related prescribed doses, DRx
t , calculated in time periods t
ranging from 1988 to 1999 using contemporaneous
共ca. 1999兲 source strength standards and dosimetry paramTx
, based upon an updated
eters, to administered doses, D99
dose-rate constant and the National Institute of Standards
and Technology 共NIST兲 primary air-kerma strength standard,
SK,N99, using the wide-angle free-air chamber 共WAFAC兲2
which had been implemented on 1 January, 1999. The updated dose-rate constant for the Model 200 103Pd seed
共TheraSeed®兲 was obtained by averaging a TLD
measurement3 with a value derived from Monte Carlo
simulation.4 The AAPM 2000 report concluded that for doses
of 115 Gy prescribed in the periods 1988–1997 and 1997–
1999, the corresponding administered doses were 124 and
135 Gy, respectively. Based on the AAPM 2000 recommendations, the American Brachytherapy Society5 recommended
that the standard prescribed dose of 115 Gy for definitive
treatment of prostate cancer using 103Pd brachytherapy alone
be adjusted to 125 Gy.
This report presents updated guidance from the AAPM on
the issue of 103Pd and 125I brachytherapy dose reconstruction,
and was prepared by the AAPM Photon-Emitting Brachytherapy Dosimetry 共PEBD兲 Subcommittee 共Chair, J. Williamson兲 and approved by the AAPM Radiation Therapy
Committee and Science Council. Because several unanticipated developments occurring after the publication of the
2000 recommendations1 impacted its recommended dose ratios by more than 5%, PEBD believed that the issue of prescribed dose selection for 103Pd brachytherapy needed to be
revisited. These developments include:
• Identifying and correcting a 5.3% error in NIST
SK,N99 calibration measurements performed in 1999
for the Model 200 source.
• Subsequent revisions of dosimetry parameters, most
notably the one-dimensional 共1D兲 anisotropy function.
• Recent revisions in the 1D dose-calculation formalism recommended by AAPM,6 resulting in replacement of the anisotropy constant by the 1D anisotropy
function.
• Publication4,7 of reference-quality Monte Carlo
single-source dosimetry parameters that distinguish
between the “heavy” seed 共low specific-activity
reactor-produced radioactive palladium兲 and “light”
seed 共higher specific-activity accelerator-produced
radioactive palladium兲 versions of the Model 200
source. These publications indicated a small change
共1.2%兲 in the dose-rate constant and a 2.3% change
in the anisotropy constant.
Medical Physics, Vol. 32, No. 5, May 2005
1425
• An improved formalism for estimating 共DTx / DRx兲t
ratios.
In contrast to 103Pd brachytherapy, no significant changes
were anticipated for 125I implant dosimetry. AAPM guidance
last addressed the issue of dose prescription for 125I implants
in 1998.8 The AAPM recommended that clinics reduce the
prescribed dose for 125I implant monotherapy from 160 to
144 Gy upon simultaneously adopting dosimetric parameters
recommended by the 1995 TG-43 report9 and implementing
the NIST 1999 SK primary standard. Since implementation of
this standard in 1999, no significant shifts in source strength
for this source model have occurred. In particular, the vendor’s source strength calibration procedures for the Model
6711 source were not affected by the NIST measurement
anomalies of 1999. The revised TG-43 dose-calculation formalism and Model 6711 dosimetry parameters published in
20046 did not significantly alter the single-seed dose-rate distribution for this source. However, because of the 2004
changes in 125I recommended dose-calculation practice and
the modified methodology for estimating 共DTx / DRx兲t ratios
presented in this report, PEBD believed it was necessary to
reevaluate dose ratios for 125I as well as 103Pd brachytherapy.
II. METHODS AND MATERIALS
A. Brief history of
103
Pd brachytherapy dosimetry
The history of 103Pd brachytherapy dosimetry is intimately related to that of the first 103Pd interstitial source
product, Theragenics Corporation’s Model 200 TheraSeed®,
introduced to the market in 1987. The early evaluated clinical experience, published by Prestidge et al.10 in 1997, was
based upon patients treated with the Model 200 source during the period 1988–1994. Later in 2000, Sharkey et al.11
reported the clinical experience with 1048 patients with 103Pd
implants treated from 1991 to 1999. Thus for clinicians practicing today who wish to reproduce the doses prescribed by
these investigators, knowing the equivalent dose to deliver,
based upon currently recommended dose-computation and
calibration practices, is essential, regardless of what commercial 103Pd seed product they choose to use. Hence the
dosimetric history of 103Pd brachytherapy is equivalent to
that of the Model 200 commercial product.
In the following sections, important events in the history
of the Model 200 source dosimetry and development of airkerma strength 共SK兲 standard are reviewed.
1. Theragenics™ calibration standard „1988–1997…
Prior to the implementation of the 1999 NIST WAFAC
standard, a primary SK standard was not available for 103Pd
or any other low-energy interstitial seed with the exceptions
of the 3M 共now Amersham Health兲 125I seeds, Models 6701,
6702, and 6711.12 Thus, Theragenics™ developed a method
for measuring apparent activity using a NaI共Tl兲 scintillation
detector, which compared Model 200 103Pd seed photon
emission rate with the 22 keV emission line 共the average
energy of the 103Pd seed emission spectrum兲 from a 109Cd
Era
1993⬍ t 艋 present
Radial dose function
Reference dosimetry data
Anisotropy function
Comments
⌳02D,N99S = 0.694
gL,02D共r兲
Monroe and Williamson 2002
␾an,04D共r兲
¯ an,04D = 0.884
␾
Monroe–Williamson 2002
Heavy seed era
No updated TG-43
report recommendations
⌳04D,N99S = 0.686
2004 TG-43 report
recommendation
Average of Nath and Monroe
gL,04D共r兲
Monroe and Williamson 2002
as recommended by
TG-43 2004
␾an,04D共r兲
¯ an,04D = 0.862
␾
Monroe-Williamson data per 2004 TG-43
recommendation
Light seed era Equation 共6兲 used
Prescription dosimetry data
Equation 共11兲 assumed
3 / 00⬍ t 艋 3 / 01, 00D dosimetry
AAPM 2000 Report
recommendations
⌳00D,N99S = 0.665
g P,95D共r兲
¯ an,95D = 0.90
␾
Equation 共11兲 assumed
⌳00D,N99S based on CY 1999 SK,N99 calibrations.
Continued use of TG 43 1995
relative dose functions
recommended
3 / 01艋 t 艋 present
Theragenics implements corrected
WAFAC standard
⌳01D,N99S = 0.68
g P,95D共r兲
¯ an,95D = 0.90
␾
Unchanged from
above except that
measured ⌳00D,N99Sadjusted by 5%.
t ⬎ tTG43U1 , 04D dosimetry, TG-43 2004
¯ an,04D andgL,04D
Report data but using ␾
⌳04D,N99S = 0.686
g P,04D共r兲
G P共r兲
¯ an,04D = 0.862
␾
Equation 共11兲 assumed
t ⬎ tTG43U1 , 04D dosimetry, TG-43 2004
Report data but ␾an,04D共r兲 and gL,04D
⌳04D,N99S = 0.686
gL,04D共r兲
GL共r兲
␾an,04D共r兲
Equation 共6兲 assumed
I interstitial brachytherapy
¯ an,95D = 0.90
␾
125
g P,95D共r兲
Pd and
⌳95D,T88S = 0.74
103
1988⬍ t 艋 3 / 00, 95D dosimetry TG-43
1995 report recommendations
Williamson et al.: Dose specification for
1988⬍ t 艋 1993
Dose-rate constant
Pd Source. tTG43U1⬎ March 2004 denotes the date on which the revised TG-43 recommendations were implemented.
103
1426
Medical Physics, Vol. 32, No. 5, May 2005
TABLE I. Prescription and reference dose calculation parameters for the model 200
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Williamson et al.: Dose specification for
103
Pd and
125
I interstitial brachytherapy
1427
Medical Physics, Vol. 32, No. 5, May 2005
1.000
0.975
0.957
0.979
1.000
1.000
t ⬎ tTG43U1 04D dosimetry parameters and recommended
1D formalism Equation 共6兲
1.000
0.980
1.000
t ⬎ tTG43U1 04D dosimetry parameters 1995 TG-43
1D formalism Equation 共11兲
0.996
0.957
0.966
3 / 01⬍ t 艋 tTG43U1 ⌳ adjusted for 2000 WAFAC
correction
0.953
0.979
0.987
3 / 00⬍ t 艋 3 / 01 AAPM 2000 dosimetry parameters
0.974
0.880
0.887
0.876
0.911
D90
0.907
1993⬍ t 艋 3 / 00 Light seed era
95D dosimetry parameters
where the exposure-rate constant for 103Pd, 共⌫␦兲x, and mean
energy expended per ion pair created, 共W / e兲, take the numerical values recommended by the AAPM.13
Because of the 463.3 day half life of 109Cd,14 four successive calibration sources were used during the period 1988–
1997. Pairwise sequential intercomparisons between the old
and replacement standards permitted changes in SK,Tnn relative to SK,N99 to be reconstructed.1 Prior to implementing the
T97 calibration source in October 1997, these variations
were less than 2%.
0.921
共1兲
1988⬍ t 艋 1993 Heavy seed era
95D dosimetry parameters
SK,Tnn = Aapp,Tnn · 共⌫␦兲x · 共W/e兲
Era
calibration standard, which had a NIST-traceable activity
calibration with an assigned uncertainty of ⫾ 5%. More details are given in the 2000 report.1 The resultant apparent
activity, Aapp,T88, denotes the quantity measured by Theragenics™ assay, where the “T” of the subscript “Tnn” denotes
Theragenics™ and “nn” denotes the year that the 109Cd standard, to which the measurement is traceable, e.g., 1992 for
the “T92” 109Cd standard, was implemented. Note that
Aapp,Tnn is fundamentally different from apparent activity as
defined by the AAPM,13 Aapp,N99, which is a quantity derived
from NIST’s 1999 standard, SK,N99. The vendor’s apparent
activity assay can be related to the vendor’s air-kerma
strength by
= Aapp,Tnn · 1.293 ␮Gy · m2 · h−1 · mCi−1 ,
0.880
D60
Clinical implant averaging
“Dummy” radial dose function, g⬘(r)
0.986
1.014
1.338
1.347
1.520
1.525
1.502
1.502
1.403
1.412
1.288
1.284
1.229
1.228
1.143
1.143
1.107
1.114
1.000
1.000
0.762
0.761
0.572
0.571
0.426
0.426
0.318
0.316
0.235
0.235
0.174
0.173
0.095 9
0.094 4
0.052 9
0.051 8
0.033 0
0.028 4
0.023 1
0.022 3
0.007 35
0.006 70
G共r兲- weighted single-seed
approximation
0.686
0.855
RDF equivalence
approximation
0.694
0.875
Averaging approximation
Distance (cm)
0.10
0.15
0.25
0.30
0.40
0.50
0.60
0.75
0.80
1.00
1.50
2.00
2.50
3.00
3.50
4.00
5.00
6.00
7.00
7.50
10.00
Light seed
TABLE III. Prescription and reference dose calculation parameters. tTG43U1⬎ March 2004 denotes the date on which the revised TG-43 recommendations were implemented.
⌳
¯⬘
“Dummy” ␾
an
Heavy seed
0.912
TABLE II. Dummy TG-43 dose calculation parameters for the Model 200
Pd-103 source.
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Williamson et al.: Dose specification for
103
Pd and
125
I interstitial brachytherapy
4. Shift in vendor calibration „Fall 1997…
2. Early single-source dosimetry parameters
„1990–1995…
During the 1988–1990 time period, both Meigooni et al.15
and Chiu-Tsao et al.16 used TLD dosimetry to measure the
dose-rate constant and relative two dimensional 共2D兲 dose
distribution parameters. These investigators measured the absolute dose rate at 1 cm on the transverse axis, and normalized this measurement to the SK,T88 inferred from the vendor’s Aapp,T88 value to obtain an estimate of the dose-rate
constant. The dose-rate constant recommended by the 1995
AAPM TG-43 report9 took the average of these two measurements and applied a multiplicative correction 共1.048兲 to
convert from Solid Water® measurement medium to a liquid
water reference phantom,
⌳95D,T88S =
D95D共r = 1 cm, ␪ = ␲/2兲
SK,T88
= 0.74 cGy · h−1 · U−1 .
共2兲
The first subscript of ⌳95D,T88S, 95D, refers to the date of the
publication documenting the measured dose rate at 1 cm,
D95D共r = 1 cm, ␪ = ␲ / 2兲, 共in this case, the 1995 TG-43 report兲, while the second subscript, terminating in an “S” for
“strength,” refers to the source calibration standard, to which
the dose rate is assumed to be normalized. Thus, it is assumed that clinical investigators, whose subsequent reports
define the clinical experience supporting 103Pd prostate
brachytherapy, utilized dose-calculation parameters equivalent to those tabulated in the 1995 TG-43 report.
3. Transition from “heavy” to “light” seed design
„1992–1993…
The above-described TLD measurements were performed
using seeds containing low specific activity reactor-produced
103
Pd. These seeds, called “heavy seeds,” were gradually replaced with “light seeds,” containing higher specific-activity,
accelerator-produced 103Pd, over a one-year period ending in
early 1993. Each Model 200 seed contains two graphite pellets with palladium metal coatings within which the radioactivity is uniformly distributed. Monroe and Williamson7 approximated the effect of the heavy-to-light seed
manufacturing process modification on seed geometry by reducing the thickness of this metal coating from 10.5 µm 共260
µg Pd/pellet兲 to 2.2 µm 共57 µg Pd/pellet兲 in their simulations.
Using Monte Carlo techniques, these authors found that the
dose-rate constant 共when normalized to the WAFAC standard兲 and radial dose function were not significantly affected
¯ an 关based
by this change. However, the anisotropy constant ␾
on an inverse-square law weighted average of ␾an共r兲 over the
1 to 5 cm distance range兴 was found to be 0.884 and 0.862
for the “heavy” and “light” seed designs, respectively. Monroe and Williamson7 provided a preliminary evaluation of
this effect on 共DTx / DRx兲t ratio using the results of their
Monte Carlo analysis of the Model 200 seed.
Medical Physics, Vol. 32, No. 5, May 2005
1428
In contrast to previous 109Cd standard replacements, the
replacement implemented by Theragenics Corporation in
fall, 1997 resulted in a 9.7% decrease in apparent activity
assays relative to SK,T94 共column 4, rows 3 and 4 of Table
IV兲, corresponding to the decrease in Aapp initially observed
by several physicists in 1997. The apparent activities and
nominal air-kerma strengths traceable to this standard are
denoted by Aapp,T97 and SK,T97, respectively. Relative to the
time-weighted 1988–1997 average of the four prior SK,Tnn
standards, S̄K,T88–94, SK,T97 calibration values are 9% smaller:
SK,T97 / S̄K,T88–94 = 0.911. These data indicate that the Theragenics™ assay was essentially constant from 1988 until Fall
1997. In 2000, Theragenics amended their calibration procedure to ensure that their Aapp calibration will be maintained
within ⫾ 2% of its post-1997 level following future 109Cd
source standard replacements.
5. Implementation of the NIST WAFAC 1999
standard
Based on measurements performed in 1998 and 1999, a
new SK standard for Model 200 source air-kerma strength,
SK,N99, was established by NIST in 1999 based upon the
WAFAC.2 A difference of more than 23% between Theragenics’ SK,T97 assay and the NIST WAFAC SK,N99 values was
noted. The conversion factor relating the two definitions was
determined to be:
SK,vendor SK,T97
=
= 0.767 ± 0.006.
SK,NIST SK,N99
The 共DTx / DRx兲t ratios recommended by the AAPM 2000 report were based upon this value. Theragenics began to issue
calibration certificates traceable to SK,N99 on 20 March, 2000.
6. Revised dosimetry parameters and AAPM 2000
dose-specification guidance „1999–2000…
In preparation for implementing the SK,N99 standard, Theragenics commissioned two dose-rate constant determinations
for the Model 200 source. Using TLD dosimeters in a solid
water phantom, Nath et al.3 reported a dose-rate constant,
⌳00D,N99S, value of 0.65± 0.05 cGy h−1 U−1. Williamson4 reported a value of 0.68± 0.02 cGy h−1 U−1 using Monte Carlo
simulation techniques. Both values were traceable to the
WAFAC standard as implemented in calendar year 1999. The
AAPM 2000 guidance document recommended using an
equally weighted average of these two values yielding
⌳00D,N99S = 0.665± 0.03 cGy h−1 U−1. That report recommended that the relative dosimetry parameters, i.e., radial
dose function and anisotropy constant, given in the 1995
TG-43 report continue to be used. These parameters are designated by the subscript “00D.”
7. Discovery and correction of calendar year 1999
WAFAC measurement errors „March 2001…
Due to an unresolved anomaly, WAFAC calibrations performed at NIST in calendar year 共CY兲 1999 were systemati-
Initiating event
SK,t Ⲑ SK,N99
Ⲑ
具D共r៝兲典xref 具D共r៝兲典xRx
Tx
共Dt兲Rx
5/88–3/90
Heavy seed production 共Cd source #1兲
95D, T88S
0.877
4/90–3/93
Heavy seed production 共Cd source #2兲
95D, T90S
0.890
1.024 共1.048兲a 关1.064兴
4/93–11/93
End of heavy seed era: light seed production begins 共Cd
source #2兲
95D, T90S
0.890
0.989 共1.010兲a 关1.064兴b
12/93–6/94
Light seed production 共Cd source #3兲
95D, T93S
0.861
7/94–9/22/97
Light seed production 共Cd source #4兲
95D, T94S
0.894
9/22/97–3/19/00
SK,T97 standard shifts by 9.7%, 共Cd source #5兲
95D, T97S
0.807
0.880 关0.899兴
3/20/00–3/4/01
Theragenics replaces SK,T97 with SK,N99 and AAPM 2000
dose parameters 00D, N99S accepted
00D, N99S
1.053
3/5/01–tTG43U1
Corrected SK,N99 standard implemented and DRC
revised
01D, N99S
⬎tTG43U1
04D, N99S parameters
⬎tTG43U1
04D, N99S parameters
1.022 共1.010兲a 关1.099兴b
Tx
共Dt兲Rx
= 1.029 DxTx = 118 Gy
其
Tx
共Dt兲Rx
= 0.990 DxTx = 114 Gy
119.4
115
117.7
115
113.7
115
117.5
115
113.2
115
b
125
115
0.979 关1.000兴b
0.930 共0.940兲a 关1.000兴b
116
125
1.00
0.957
0.957 共0.990兲a
120
125
125
0.984 共1.010兲a 关1.059兴b
其
Pd and
04D, N99S
共approved 1D formalism兲
1.00
1.000
1.000 共1.000兲a
125
125
04D, N99S
共old 1D formalism兲
1.00
0.980 共1.000兲a
0.980 共1.000兲
122
125
b
1.090 共1.102兲 关1.172兴
a
I interstitial brachytherapy
Ratios from original Monroe and Williamson paper.
Ratios from AAPM 2000 Guidance.
b
0.880 关0.899兴b
1.039 共1.048兲a 关1.079兴b
DxRx
共Gy兲
103
a
0.911 关0.899兴b
DxTx
共Gy兲
Williamson et al.: Dose specification for
Time period 共t兲
Prescription
parameters
kkD, yyS
assumed by
DRx
t
1429
Medical Physics, Vol. 32, No. 5, May 2005
Tx
TABLE IV. Ratios of administered-to-prescribed dose, 共Dt兲Rx
, as a function of year t and dosimetry data tD for 103Pd Implants. Bold indicates current analysis, using CIA D90 values. tTG43U1⬎ March 2004 denotes the
date on which the revised TG-43 recommendations were implemented.
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Williamson et al.: Dose specification for
103
Pd and
125
I interstitial brachytherapy
cally elevated by seed model-dependent factors of up to 7%,
relative to measurements performed before and after this period. Many models of 125I and 103Pd sources were unfortunately affected by this anomaly. For the Model 200 source,
the SK,N99 measurements were elevated by 5.3% during this
period 共see Appendix A, Sec. 6, and Appendix B, Sec. 2.3 of
the TG-43 2004 update6兲. Unfortunately, the AAPM 20001
recommendations were based upon these erroneous measurements. This measurement error was corrected by NIST in the
first quarter of 2000.
As with other seed models, PEBD coordinated the transition to the corrected WAFAC standard with NIST, Theragenics, and the ADCLs on 5 March, 2001. Since each such transition was vendor- and model-specific, no AAPM report was
published to advise the community and vendors were responsible for communicating the relevant action plan to their clients. For the Model 200 source, the corrected WAFAC standard was implemented simultaneously by the vendor, NIST,
and ADCLs on 5 March, 2001 共Appendix A, Sec. 6 of the
TG-43 2004 update6兲. As part of this process, the measured
dose-rate constant of Nath et al.,3 which was normalized to
erroneous WAFAC measurements, was increased by 5.3% to
compensate for the correction. This led to a new revised
average dose-rate constant ⌳01D,N99S = 0.68 cGy h−1 · U−1. At
that time, AAPM advised the community to continue using
the 2000 administered-to-prescribed dose ratios and guidance derived therefrom until further notice. It is assumed that
these recommendations remain operative up to the present
time.
8. Revised dosimetry parameters and the updated
TG-43 protocol „March 2004…
Since implementation of the 2000 AAPM guidance
report,1 the Model 200 dosimetry parameters have been further refined. A comprehensive Monte Carlo-based study7 was
published by Monroe and Williamson, which contained
TG-43 dosimetry parameters for both the “light” and
“heavy” seed designs. In addition, Yue and Nath17 published
measured light-seed 2D anisotropy functions which were in
excellent agreement with Monroe’s calculations. Both studies confirmed that the 1995 TG-439 anisotropy functions,
¯ an,95D = 0.90, significantly overestimated the
yielding a ␾
source isotropy compared to more recent publications, which
¯ an,04D = 0.862.
imply ␾
In March 2004, a major update of the TG-43 protocol was
published.6 It contained revised data, designated by the subscript “04D,” for the Model 200 seed. The recommended
⌳04D,N99S is nearly identical to ⌳01D,N99S. New radial dose
functions, 1D anisotropy functions, and 2D anisotropy functions 共based on Monroe’s simulations7兲 were recommended
for clinical use. In addition, the dose-calculation formalism
itself was modified. The revised formalism requires use of
the 1D anisotropy function rather than the anisotropy con¯ an,04D and specifies a new method of calculating doses
stant, ␾
at small distances. It also advises users to adopt the linesource geometry function over the point-source function.
Both the new relative “04D” dosimetry parameters and reMedical Physics, Vol. 32, No. 5, May 2005
1430
vised dose-calculation formalism are used in the analysis to
follow as “reference dosimetry parameters,” i.e., used to estimate “administered dose.”
The dose ratios recommended in the AAPM 2000 report1
take into account the developments described in Secs. II A 1,
II A 2, II A 4, II A 5, and II A 6 above. The revised analysis,
presented in the following, takes into account the remaining
phenomena: heavy- versus light-seed design 共Sec. II A 3兲,
1999 WAFAC errors 共Sec. II A 7兲, and dosimetry parameters
and formalism revised as described by the new TG-43 protocol 共Sec. II A 8兲.
B. Generalized formalism for evaluation of
administered-to-prescribed dose ratios
To evaluate the ratio of administered dose, DTx, to prescribed dose, DRx, the methodology described by Monroe
and Williamson7 has been adopted. Their analysis accounts
for changes in the dose-rate constant used for treatment planning and time-dependent discrepancies between vendor and
NIST source-strength specifications as does the original
AAPM 2000 analysis. In addition, Monroe and Williamson7
accounted for the dosimetric effect of Model 200 seed internal geometry changes caused by the transition from the
heavy seed to the light seed production process, the NIST
1999 WAFAC anomaly, and the dose-parameter revisions
published in the 2004 TG43 protocol, none of which were
anticipated by the AAPM 2000 Report.1 The influence of
seed manufacturing process changes on the DTx / DRx ratio
was incorporated into the analysis by introducing separate
time-dependent reference dosimetry parameters for the light
and heavy seeds. The current report adapts the Monroe–
Williamson analysis with the addition of more sophisticated
dose-averaging techniques. In addition, this report consistently uses the 1D dose-calculation formalism recommended
by the 2004 AAPM report6 in contrast to the Monroe–
Williamson analysis which used the old TG43 dosecalculation formalism.9
Tx
The mean administered-to-prescribed dose ratio, 共Dt兲Rx
, is
given by
Tx
=
共Dt兲Rx
冋冓 冔册 冋
៝兲
Dref
t 共r
Rx
Dt 共r៝兲
·
册
SK,N99 t⬘ ⬎ 3/01
,
SK,t
共3兲
where t and t⬘ refer to the past time in question and current
Tx
time, respectively; 共Dt兲Rx
denotes the administered-toprescribed dose ratio for time t, and D共r៝兲 denotes the calculated dose at position r៝ in an implant. The bracketed quantity,
具X典, denotes the result of spatially averaging the indicated
quantity over the appropriate region within the planning target volume 共PTV兲 of a typical implant. Various approaches
to spatial averaging are discussed in the following. The quan៝兲 denotes the administered dose at location r៝, or
tity Dref
t 共r
dose actually delivered, during the period t as approximated
by the selected reference dosimetry parameters. Reference
parameters are those considered, based upon current knowledge, to provide the most accurate and physically rigorous
method of retrospectively and prospectively calculating dose
៝兲 denotes the corresponding
in an implant. The quantity DRx
t 共r
1431
Williamson et al.: Dose specification for
103
125
Pd and
I interstitial brachytherapy
prescribed dose derived from the dosimetry parameters in
use at time t. The last factor on the right of Eq. 共3兲 is the ratio
of “true” air-kerma strength 共SK,N99 as implemented by Theragenics after March 2001 or as implemented in January 1999
for Model 6711 sources兲 to the source-strength standard, SK,t,
accepted as definitive during the time period t for seeds of
identical physical construction emitting identical quantities
of radiation. Assuming that the new TG-43 report was implemented at time tTG43U1 ⬎ 3 / 04, Eq. 共3兲 can be used to derive
the prescribed dose, DtRx
, for use with the reference
⬘⬎tTG43U1
dosimetry parameters and the current air-kerma strength
standard that ensures that patients will continue to receive
the same delivered dose 共as estimated by retrospective application of reference dosimetry parameters兲 as otherwise identical patients planned with “t-era” dose distributions, source
strength
standards,
and
prescribed
doses:
Tx
Rx
៝
共r៝ 兩 SK,N99 , 04D , DtRx
兲
=
D
共r
兩
S
,
tD
,
D
DtTx⬘⬎t
K,t
t
t 兲.
⬘⬎tTG43U1
TG43U1
Hence
Rx
Dt⬘⬎t
TG43U1
Tx
= 共Dt兲Rx
· DRx
t ,
共4兲
where DRx
t is the prescribed dose 共in units of Gy兲 and the
arguments to the right of the vertical line indicate the dosimetric data and SK standard used for treatment planning used
at time t. Generally, the clinical goal is to reproduce the
clinical outcomes of a previously treated group of patients in
the face of significant dose-calculation and source-strength
standard revisions. Obviously, it is necessary to carefully
match the prescribed dose, dose-calculation formalism and
parameters, and SK standardization procedures to the clinical
experience one is trying to duplicate. In the sections to follow, each factor of Eq. 共3兲, along with its method of evaluation, will be defined.
C. Air-kerma strength standard revisions
The air-kerma strength ratios, SK,t / SK,N99, used in the
analysis of 103Pd seed dosimetry are given by
SK,t
SK,N99
D. Reference dosimetry parameters
For the currently available “light” seed, the parameters
recommended by the TG-43 2004 update6 were used. The
radial dose function, gL,04D共r兲, recommended therein was derived from the Monte Carlo study by Monroe and
Williamson.7 The function gL,04D共r兲 was defined over the distance range 0.1–10 cm and the 1D anisotropy function,
␾an,04D共r兲, over the distance range 0.25 to 10 cm. The recommended dose rate constant, ⌳04D,N99S, was obtained by
averaging the measured value3 共corrected for the 1999
anomaly in SK,N99兲 with the corresponding Monte Carlo
estimate.7 These data are summarized in Table I.
For the “heavy” seed, the 2004 TG-43 report makes no
recommendations regarding dosimetry parameters. The relative Monte Carlo data by Monroe and Williamson7 are assumed, an approach consistent with the AAPM consensus
data-formation methodology. This methodology offers two
choices for estimating the consensus heavy seed dose-rate
constant: 共i兲 average the Monroe–Williamson Monte Carlo
MC
value 共⌳02D,N99S
= 0.694兲 with the average of the Chiu–Tsao16
15
¯ TLD
and Meigooni measurements 共⌳
90D,N99S = 0.650兲 or 共ii兲 reject the experimental measurements as candidate data sets
and use the Monte Carlo value without modification:
MC
= 0.694. Because the Chiu–Tsao and
⌳04D,N99S = ⌳02D,N99S
Meigooni measurements were normalized to source-strength
measurements that are not traceable to the current NIST SK
standard and because these pioneering works do not adhere
to modern standards of experimental dosimetry,6 the AAPM
believes that using the unmodified Monte Carlo dose-rate
constant, i.e., option 共ii兲, provides the least uncertain estimate of the heavy seed reference dose-rate constant consistent with the AAPM consensus-formation methodology.
Reference administered doses were calculated according
to the 1D dose calculation formalism recommended by the
2004 TG-43 report 共Eq. 共11兲兲兴.6 For a single seed, the refer៝兲, is given by
ence dose rate, Ḋref
t 共r
៝兲
Ḋref
t 共r
= SK,N99 · ⌳04D,N99S ·
=
冦
0.886, t 艋 9/97,
1.053, 3/00 ⬍ t 艋 3/01, SK,t = SK,N99 in 1999
SK,t = SK,N99 in 2000
共5兲
These values were based on the ratios given in the AAPM
2000 report1 corrected by the magnitude 共1.053兲 of the 1999
WAFAC measurement anomaly, which resulted in elevated
Model 200 seed calibrations by Theragenics and the ADCLs
during the period 3 / 00⬍ t 艋 3 / 01. For the period 1988 to
September 1997, our analysis uses the SK,t / SK,N99 ratio specific to each cadmium calibration source actually used 共see
Table IV兲, not the average ratio, S̄K.T88–94 / SK,N99, indicated
by Eq. 共5兲.
Medical Physics, Vol. 32, No. 5, May 2005
GL共r, ␪0兲
· gL,04D共r兲 · ␾an,04D共r兲.
GL共r0, ␪0兲
共6兲
SK,t = S̄K,T88–94
0.807, 9/97 ⬍ t 艋 3/00, SK,t = SK,T97
1.000, t ⬎ 3/01,
1431
Equation 共6兲 was implemented on a commercial treatment
planning system 共VariSeed Planning Workstaion, Version
7.1, Varian Medical Corporation, Inc., Palo Alto, CA兲 for
permanent seed implants. However, this treatment planning
system, like many others, does not support the implementation of Eq. 共6兲 since it allows only the point-source geometry
function to be used in its implementation of the 1D TG43
formalism. PEBD notes that this planning system, as well as
many other commercial systems, would have supported
implementation of the allowed but not recommended 1D formalism 关Eq. 共10兲 of the 2004 TG-43 protocol, using G P共r兲
rather than GL共r , ␪0兲兴. This option closely approximates Eq.
共6兲 although it is less accurate at small distances, e.g., r
⬍ 1 cm. To implement Eq. 共6兲, it was necessary to “fool” the
1432
103
Williamson et al.: Dose specification for
Pd and
125
I interstitial brachytherapy
planning system’s algorithm into performing the new calculations using the older point-source approximation by using
dummy parameters. Essentially, the anisotropy constant can
be folded into the radial dose function, creating a dummy
radial dose function, g⬘共r兲. This was accomplished as follows:
៝兲 = SK,N99 · ⌳04D,N99S ·
Ḋref
t 共r
冉冊
r0
r
2
¯⬘ ,
· g⬘共r兲 · ␾
an
共7兲
where the primed quantities denote dummy parameters,
listed in Table II, designed to reproduce the dose rates predicted by Eq. 共6兲 down to distances of 0.1 cm using the
point-source geometry function and the now-forbidden anisotropy constant. Letting rmin be the smallest distance for
which ␾an,04D共r兲 is tabulated, these ratios are selected so as
to force Eq. 共7兲 to agree with the currently recommended
model, Eq. 共6兲, for each of the tabulated data entries.
For the case r 艌 rmin, this leads to the following equivalences:
g⬘共r兩r 艌 rmin兲 =
冉冊
GL共r, ␪0兲 ␾an,04D共r兲 r
·
GL共r0, ␪0兲 ␾an,04D共r0兲 r0
2
· gL,04D共r兲,
¯ ⬘ = ␾an,04D共r0兲
␾
an
共8兲
៝兲 = SK,t · ⌳tD,XtS ·
ḊRx
t 共r
1432
冉冊
r0
r
2
¯⬘ .
· gtD共r兲 · ␾
an,tD
共11兲
In the case of the future era t 艌 tTG43U1, ḊRx
t 共r兲 was evaluated
using Eq. 共7兲, using both the dummy parameters given by
Eqs. 共8兲 and 共10兲, 关the equivalent 1D dose calculation formula, Eq. 共6兲, preferred by the 2004 TG-43 report兴, and the
following parameters: ⌳tD,XtS = ⌳04D,N99S, gtD共r兲 = gL,04D共r兲
7
¯ ⬘ =␾
¯ an,04D, where ␾
¯⬘
and ␾
an,tD
an,04D = 0.862. The latter option
is equivalent to using the anisotropy constant-based 1D formalism given by Eq. D1 of the 2004 TG-43 report, a formalism widely used in the past but no longer endorsed by the
AAPM. The dosimetric parameters assumed for various eras
are summarized in Table I. This report assumes that 1995
TG-43 compatible parameters were used throughout the era
1988–2000 even though the TG-43 report was published in
1995, the same assumption made by the AAPM 2000 report.1
The 1995 TG-43 report recommendations were based upon
averaging the two measured dose-rate constants published at
that time.15,16 Those readers who wish to duplicate a particular institutional implant experience based on pre-1995 implants should confirm that the prescription parameters assumed by this report are reasonable approximations to the
institution’s dose-calculation procedures.
In the case of r ⬍ rmin, we set Eq. 共6兲 to the short-distance
extrapolation formula found in Appendix C of the 2004
TG-43 report:
Ḋ共r៝兲 = SK · ⌳ ·
冉 冊
rmin
r
2
·
GL共rmin, ␪0兲
· gL共r兲 · ␾an共rmin兲.
GL共r0, ␪0兲
F. Dose-averaging procedures
共9兲
This leads to the following dummy parameter definition for
r ⬍ rmin:
g⬘共r兩r ⬍ rmin兲
=
冉 冊
GL共rmin, ␪0兲 rmin
·
GL共r0, ␪0兲
r0
2
· gL,04D共r兲 ·
␾an,04D共rmin兲
,
␾an,04D共r0兲
¯ ⬘ = ␾an,04D共r0兲.
␾
an
共10兲
To evaluate the dummy quantities defined by Eqs. 共8兲 and
共10兲, the tabulated 1D anisotropy functions6,7 were interpolated onto the finer radial dose function grid by applying
linear interpolation to the quantity r2 · GL共r , ␪0兲 · ␾an,04D共r兲
and then converting the result back to ␾an,04D共r兲. This proce¯ an
dure is based upon Williamson’s approximation18 ␾
⬇ r2 · GL共r , ␪0兲 · ␾an,04D共r兲. The resultant values of g⬘共r兲 and
¯ ⬘ are given in Table II.
␾
an
E. Prescription dosimetry parameters
៝兲,
ḊRx
t 共r
The prescribed dose-rate distribution,
for all times
other than t 艌 tTG43U1, was assumed to have been derived
from the original TG43 point-source dose-calculation formalism:
Medical Physics, Vol. 32, No. 5, May 2005
៝兲典 and
Three different approaches to evaluating 具DRx
t 共r
were investigated by this report. In ascending order
of complexity, these approaches are called “radial dose function equivalence approximation 共RDA兲,” “geometry
function-weighted single-seed approximation 共GFSA兲,” and
“clinical implant averaging 共CIA兲.” Each of these approaches
will be described in turn.
៝兲典
具Dref
t 共r
1. Radial dose function equivalence approximation
„RDA…
The RDA approach has been used by most administeredto-prescribed dose ratio analyses published to date, including
the AAPM 2000 Report1 and the Monroe–Williamson
article.7 RDA assumes that Eqs. 共6兲 and 共11兲 yield equivalent
dose-rate predictions and that gt共r兲 ⬇ g95D共r兲. In addition, it
ignores other subtleties such as errors arising from mixing
G P共r兲 and gL共r兲 data in the same equation. Given these assumptions, mean dose ratio assumes a very simple form:
冓 冔
៝兲
Dref
t 共r
Rx
Dt 共r៝兲
=
共⌳04D,N99S · ␾¯ an,04D兲ref
.
共⌳tD,XtS · ␾¯ an,t兲Rx
共12兲
For the Model 200 seed, using the data from Table I we
obtain
1433
Williamson et al.: Dose specification for
103
Pd and
125
I interstitial brachytherapy
៝兲典
具DRx
t 共r
N
具f典G共r兲 =
¯ an兲Rx
= 共⌳ · ␾
t
=
冦
៝兲典
具Dref
t 共r
¯ an兲95D,T88S , t 艋 3/00
0.74 · 0.90 = 共⌳ · ␾
¯ an兲00D,N99S , 3/00 ⬍ t 艋 3/01
0.665 · 0.90 = 共⌳ · ␾
¯ an兲01D,N99S , 3/01 ⬍ t 艋 tTG43U1
0.68 · 0.90 = 共⌳ · ␾
ri艌1 cm
冓 冔
៝兲
Dref
t 共r
Rx
Dt 共r៝兲
0.694 · 0.884 heavy seed, t 艋 1993
0.686 · 0.862 light seed,
t ⬎ 1993.
共13兲
2. Geometry-function weighted single-seed
approximation „GFSA…
The simple RDA approximation is not consistent with the
2004 TG-43 report guidance, which recommends using the
1D anisotropy function over the anisotropy constant. Nor
does it account for the differences between g95D共r兲 and
g04D共r兲 data recommended by the new report. To accommodate these changes, a generalized single-seed averaging procedure was explored, defined by
¯ an,t具gt共r兲典共r−2兲 =
= ⌳tD,XtS · ␾
=
=
¯ an兲Ref
= 共⌳ · ␾
t
再
冦
ri艌1 cm
G共ri兲,
៝兲典
具Dref
t 共r
Rx
具Dt 共r៝兲典
⌳04D,N99S具␾an,04D共r兲 · gL,04D共r兲典GL共r兲
¯ an,t具gt共r兲典共r−2兲
⌳tD,XtS · ␾
=
再
共15兲
0.694 · 0.6554 = 0.4548 heavy seed, t 艋 1993
0.686 · 0.6399 = 0.4390 light seed,
t ⬎ 1993.
共16兲
Since the prescribed dose distribution is calculated by Eq.
៝兲典 becomes
共11兲, then 具DRx
t 共r
95D,
t 艋 3/00
3/00 ⬍ t 艋 3/01
0.68 · 0.90 · 0.7529 = 0.4608,
01D,
3/01 ⬍ t 艋 tTG43U1
0.686 · 0.862 · 7483 = 0.4425,
04Dg P,04D共r兲, t ⬎ tTG43U1
0.686 · 0.862 · 7455 = 0.4408,
04DgL,04D共r兲, t ⬎ tTG43U1 ,
The most accurate and appropriate approach to dose averaging is to implement reference and prescription dosecalculation models on a brachytherapy treatment planning
system using the geometry from typical clinical implants to
assess the change in typical prescription parameters.19 This
method is referred to as clinical implant averaging 共CIA兲. To
implement CIA, the seed geometry from four typical clinical
103
Pd implants was used. The implants consisted of prostate
target volumes ranging from 22 to 46 cm3, prescribed D90
doses ranging from 76 to 130 Gy, and 40–72 Model 200
.
៝兲典 = ⌳04D,N99S具␾an,04D共r兲 · gL,04D共r兲典G 共r兲
具Dref
t 共r
L
0.665 · 0.90 · 0.7529 = 0.4506, 00D,
3. Clinical implant averaging
共14兲
This approach is identical to that recommended in 2004
TG-43 report for estimating the now-forbidden anisotropy
constant. It was found that 具f典G共r兲 was quite sensitive to the
N
grid assumed. Hence all averages, both for planned
兵ri其i=1
N
and reference doses, were based on the choice 兵ri其i=1
= 兵1 , 1.5, 2 , 2.5, 3 , 3.5, 4 , 5 cm其.
For the reference dose calculations, GFSA yields the following:
0.74 · 0.90 · 0.7529 = 0.5014,
where tTG43U1 艌 3 / 04 denotes the date on which the 2004
TG43 recommendations were implemented.
Medical Physics, Vol. 32, No. 5, May 2005
冒兺
N
f共ri兲 · G共ri兲
N
where 兵ri其i=1
denotes the set of radial distances 艌1 cm for
which g共r兲 and ␾an共r兲 are specified; G共ri兲 is the inverse
square-law weighting factor; and f共r兲 is the function to be
averaged. The mean-dose ratio is then given by
¯ an兲04D,N99S , t ⬎ tTG43U1 ,
0.686 · 0.862 = 共⌳ · ␾
=
៝兲典
具DRx
t 共r
兺
1433
共17兲
Pd seeds implanted with a modified peripheral loading.20
Source positions and the prostate CTV contours were derived
from x-ray CT examinations obtained 30 days following the
implant. The planning system 共VariSeed Planning Workstation, Version 7.1, Varian Medical Corporation, Inc., Palo
Alto, CA兲 calculated dose on a 共2 ⫻ 2 ⫻ 3兲 mm3 grid using
the “constant 共point model兲” with the option “anisotropic
correction” selected. The dose calculation was repeated using
different dosimetric parameters for the various prescription
and reference dose eras specified above. The source strength
per seed was held constant for each simulation, using the
Sk,N99/seed value assumed for the clinical treatment plan.
Dose calculations were performed using the vendor’s dosecalculation algorithm described by Eqs. 共7兲 and 共11兲. In the
103
1434
Williamson et al.: Dose specification for
103
Pd and
125
I interstitial brachytherapy
1434
៝兲 / DRx
៝兲典 and corresponding standard deviation as a function of reference dose in units of D90 for sample clinical
FIG. 1. Plots of mean dose ratios, 具Dref
t 共r
t 共r
implant No. 1. The means and standard deviations are averages of the calculation-point dose ratios falling in dose bins with widths of approximately 12 Gy.
The right upper and lower left graphs compare the 95D prescription dose parameters and formalism 关Eq. 共11兲兴 to the heavy and light seed reference 共04D兲 dose
parameters, respectively. The upper left graph compares the anisotropy constant prescription formalism 共D1 of 2004 TG-43兲 to the Eq. 共11兲 reference dose
formalism using 04D parameters in both cases.
case of the reference dosimetry calculations, the dummy parameters summarized in Table II were used, which yields
doses equivalent to Eq. 共6兲.
As our results 共see Sec. III below兲 show that the mean
dose ratio is constant within 1% for doses ranging from
0.25D90 to 1.8D90, D90 and D60 were extracted from the resultant prostate DVHs for the four patients to derive the
mean dose ratios recommended by this report:
冓 冔
៝兲
Dref
t 共r
Rx
Dt 共r៝兲
4
1
共DXX,i兲ref,t
=
,
4 i=1 共DXX,i兲Rx,t
兺
共18兲
where XX denotes either 90% or 60% of the prostate volume,
and DXX,i represents the corresponding DVH statistic from
the ith sample implant.
III. RESULTS
A. Reference-to-prescription dose ratios for
brachytherapy
103
Pd
Figure 1 illustrates the dependence of mean dose ratio, as
Medical Physics, Vol. 32, No. 5, May 2005
evaluated by CIA, on the dose in multiples of D90 for the
implant having the largest volume. For all three primary
comparisons of reference and prescription dosimetry parameters 共the others can be obtained by scaling the graphs by the
appropriate dose-rate constant ratio兲, the mean dose ratio is
virtually constant between 0.5D90 and 2D90, which includes
the peripheral layers of the target most relevant to clinical
dose prescription. Figure 2 shows the dependence of the dose
ratio on spatial position in the transverse bisecting plane of
the implant. As illustrated by Figs. 1 and 2, at doses below
0.25D90, the mean dose ratio increases. As the commercial
planning system exports dose values as 16 bit integers scaled
from zero to the maximum dose, this behavior arises from
integer truncation. The uncertainty introduced by discretization of doses is less than 0.2% in the therapeutic dose range.
Very near the seeds, Figs. 1 and 2 indicate that the dose
ratios are much larger and more variable. The
共DXX,i兲ref,t / 共DXX,i兲Rx,t ratios were found to be nearly independent of the implant geometry, i, showing a maximum range
of 0.001 over the four implants for both D60 and D90.
1435
Williamson et al.: Dose specification for
103
Pd and
125
I interstitial brachytherapy
1435
៝兲 / DRx
៝兲 as a function of position in the transverse 共top兲 and sagittal 共bottom兲 planes bisecting the center of the implanted
FIG. 2. Plots of the dose ratio, Dref
t 共r
t 共r
volume for sample clinical implant No. 1. The 95D prescription dose parameters and formalism 关Eq. 共11兲兴 are compared to the light seed reference 共04D兲 dose
parameters.
Medical Physics, Vol. 32, No. 5, May 2005
1436
Williamson et al.: Dose specification for
103
Pd and
125
I interstitial brachytherapy
1436
FIG. 3. Plot illustrating the variation of the delivered dose for prescribed doses of 115 and 125 Gy as a function of time for the Model 200 103Pd source. After
2000, delivered doses are plotted for prescribed doses of 115 Gy 共solid line兲 and 125 Gy 共broken line兲. For illustration only, we have assumed tTG43U1
= 7 / 05. For post-tTG43U1 implants, the plot assumes that the AAPM preferred 1D dose-calculation model is used 关Eq. 共6兲兴.
Table III compares the mean reference-to-prescribed dose
ratios for the RDA, GFSA, and CIA averaging methods.
¯ an- and
With the exception of one case 共comparison of ␾
␾an共r兲-based formalisms using 04D data兲, all estimates agree
within 1.5%. Generally, the RDF approximation overesti៝兲 / DRx
៝兲典 value by about 1% while
mates the CIA 具Dref
t 共r
t 共r
GFSA results in a 0.5% underestimate. For all cases, the D90
and D60 specification parameters produce virtually identical
¯ an vs ␾an共r兲 formalism
CIA estimates. However, in the ␾
comparison case, the CIA dose-ratio estimate is 1.6% and
2% smaller than the RDA and GFSA estimates, respectively.
Since the same dosimetry data are used by all three methods,
the difference between GFSA and CIA must arise from the
different formalisms, Eqs. 共6兲 and 共11兲, used in the denominator and numerator, respectively, of these two methods. The
discrepancy suggests that the r−2 weighting scheme for r
艌 1 cm gives a biased estimate of the average single-seed
calculation distance characteristic of clinical implants. In the
1988–present comparisons, this effect could have been
masked by differences in radial dose function in the prescription versus reference eras. While the discrepancies among
these three methods are small in relation to the overall unMedical Physics, Vol. 32, No. 5, May 2005
certainty of clinical dose calculation, it is prudent to recommend the clinical implant averaging technique for performing future assessments of this type.
B. Administered-to-prescribed dose ratios
Tx
The final 共Dt兲Rx
ratios recommended in this report, as estimated by the CIA technique, are listed in Table IV and
plotted in Fig. 3, along with the corresponding values from
the 2000 AAPM report1 and the Monroe paper.7 In addition,
Tx
the time-weighted average 共Dt兲Rx
ratios for the heavy seed
共1988–1993兲 and pre-9/97 light seed 共1993–1997兲 eras are
tabulated. Compared to the 2000 AAPM report, the revised
ratios for the heavy seed and pre-9/97 light seed eras are
3.8% and 7.1% smaller, respectively. Thus, a dose of 115 Gy
prescribed in the periods 1988–1993 and 1993–1997 yields
administered doses of 118 and 114 Gy, respectively, in contrast to the average administered dose of 124 Gy estimated
by AAPM in 2000.1 The revised 9 / 97 to 3 / 00 dose ratio is
7% smaller than that recommended by the 2000 Report, resulting in an administered dose of 124 Gy compared to 135
Gy. Using the revised dose ratios recommended by this re-
1437
Williamson et al.: Dose specification for
103
Pd and
125
I interstitial brachytherapy
port, a prescribed dose of 125 Gy5 delivered during the 3 / 00
to 3 / 01 era 共following WAFAC implementation by Theragenics but preceding correction for the 1999 NIST measurement anomalies兲, corresponds to a delivered dose of 116 Gy,
which is 7% lower than the 2000 Report estimate. Following
Theragenics’ implementation of the corrected SK,N99 standard
and the associated dose-rate constant adjustment in 3 / 01,
this difference was reduced to a 4% underdose, i.e., 125 Gy
prescribed corresponds to a delivery of 120 Gy. The major
causes of these revised ratios are the 5.3% error caused by
the 1999 NIST WAFAC anomaly for the Model 200 source
共which was partially mitigated by the revision of the doserate constant from 0.665 to 0.68兲 and the adoption of revised
2D anisotropy functions, which resulted in a change in anisotropy constant from 0.90 to 0.862. These two changes
were first evaluated by Monroe and Williamson7 in 2002, but
neither was anticipated by the 2000 report. For institutions
that implement the revised TG43 formalism, Eq. 共6兲, and
associated dosimetry parameters6 共next-to-last line, Table IV兲
without adjusting the prescribed dose, the delivered dose will
increase by 4%, from 120 to 125 Gy. For those who implement the revised dosimetry parameters using the old TG43
1-D formalism, Eq. 共11兲, the administered dose will increase
by 1.7%, from 120 to 122 Gy 共last line, Table IV兲.
C.
125
I Model 6711 source
As is the case with palladium, the history of iodine seed
dosimetry is closely related to the history of a single seed
model, the Amersham Model 6711 seed. It is well understood
that the original prescribed dose of 160 Gy, used from 1985
until 1995, was based on dosimetry parameters21 that were
changed by the publication of the 1995 TG-43 report. The
result was to change the prescribed dose to 144 Gy,8 and
from 1995 to the present, most institutions have followed
this change.
The manufacturer of the 6711 seed did not adopt the NIST
1999 air-kerma standard through the transfer of a calibrated
source, but instead mathematically implemented an adjustment of 0.897. Consequently, the NIST measurement
anomaly 共⫹5.1% for the Model 6711 seed兲 of 1999 that affected the vendor calibrations of the Model 200 and many
other source types did not influence Amersham calibrations
of the Model 6711 seed. Therefore, only one transition was
made in the determination of air-kerma strength of this seed;
that of the introduction of the NIST 1999 standard.
It is recognized that the incorrect NIST 1999 standard was
disseminated to the ADCLs, who may have passed it along to
customers, who may then have adjusted the strengths of
seeds delivered to them by the manufacturer, but this is not
known with any certainty. Thus, this possibility is ignored
for this analysis.
For the 6711 seed, ␾an,04D共r兲 is almost constant from 1 to
¯ an,04D, 0.943, was
5 cm, so that the anisotropy constant, ␾
used in a comparison of delivered doses via the RDA
method. This value is slightly different from the value of
0.93 recommended by the 1995 TG-43 report.9 The differences among the 83D, 95D, and 04D radial dose functions
Medical Physics, Vol. 32, No. 5, May 2005
1437
are not large. The comparison of the RDA and GFSA doserate analyses in Table V shows that these errors influence the
administered-to-prescribed dose ratios by at most 1%.
The 1995 TG-43 report recommended a dose rate constant
of 0.88, which was mathematically modified to 0.98 in 1999,
to accommodate implementation of the 1999 NIST
standard.8 The 2004 TG-43 report further revises this value
to 0.965, which almost exactly compensates for the changes
¯ an from 1995 to 2004. As a result, the changes in delivin ␾
ered dose from the introduction of the Model 6711 seed to
the present have been less than 0.5% and can safely be ignored. These data are summarized in Table V.
IV. DISCUSSION AND CONCLUSIONS
The methodology for analyzing the impact of the implementation of new dose calculation parameters, formalism,
and changes in source strength calibration standards, presented here, was developed originally by Dr. Williamson and
Dr. Todor at Virginia Commonwealth University. Later, it
was reviewed, reformulated, and incorporated into this report
by the AAPM PEBD subcommittee. This document was reviewed and approved by the AAPM PEBD Subcommittee,
Radiation Therapy Committee, and Science Council. Hence
this report represents the official position of the AAPM on
the recommendations for the impact of the implementation
of 2004 TG-43 update on the dose delivered by interstitial
brachytherapy sources.
For this analysis, we selected four typical prostate implants because the most popular application of interstitial
brachytherapy continues to be organ-confined early stage
prostate cancer. That the doses used in the actual treatment of
the four patients differ from the commonly prescribed monotherapy dose of 125 Gy for 103Pd prostate implants does not
impact this analysis, since absolute source strength and dose
do not influence the estimated dose ratios. Of the eight
sources presented in the 2004 TG-43 update, our analysis
considered only the 125I Model 6711 source and 103Pd Model
200 source because the vast majority of papers documenting
the clinical efficacy of permanent prostate brachytherapy,
representing decades of clinical experience, are based upon
these sources. Brachytherapy of prostate cancer has become
the treatment of choice for selected patients because of excellent rates of local control with minimal treatment related
toxicity. With such a successful therapeutic option, it is critical that any changes in dose delivery techniques, including
changes in dose calculation parameters and formalism, as
well as procedures used in establishing source strength,
should be analyzed critically in order not to compromise the
therapeutic implant in potentially curable patient populations. This is the motivation behind this rather densely written and complex analysis.
As mentioned earlier, the 1999 NIST anomaly affected
many interstitial brachytherapy sources, some by up to 7%.
Although dose calculations for several source models were
affected by 1999 WAFAC measurement anomalies comparable to or greater than those affecting the Model 200 seed,
these sources are not considered by our analysis. Because
1.000 共1.000兲
1.000 共1.000兲
145
145
Pd and
1.000
Current analysis, using RDA method.
04D,N99S parameters
⬎ Present
Medical Physics, Vol. 32, No. 5, May 2005
103
¯ an,04D共r兲 共old
04D,N99S, g P04D共r兲, ⌳04D,N99S = 0.965, ␾
1D formalism兲
145
145
1.000 共1.000兲
1.000 共1.000兲
1.000
¯ an,04D = 0.943
04D, N99S, gL04D共r兲, ⌳04D,N99S = 0.965, ␾
共approved 1D formalism兲
04D,N99S, parameters
⬎ Present
145
0.987 共0.998兲
1.000
1999 WAFAC measurement anomaly corrected
2000–present
¯ an,95D = 0.93
95D, N99S, g95D共r兲, ⌳95D,N99S = 0.98, ␾
0.987 共0.998兲
143.0 共144.7兲
145
0.987 共0.998兲
1.000
1995 TG-43, 95D parameters and WAFAC SK,N99
standard implemented
1999–2000
¯ an,95D = 0.93
95D, N99S, g95D共r兲, ⌳95D,N99S = 0.98, ␾
0.987 共0.998兲
143.0 共144.7兲
160
144.4 共144.5兲
1.115
Pre-TG-43 dosimetry era, Loftus SK,N85 standard
1985–1999
¯ an,83D = 0.87
83D, N85S, g83D共r兲, ⌳83D,N85S = 1.039, ␾
0.902 共0.903兲a
Ⲑ
Time period, 共t兲
Initiating event
SK,t / SK,N99
1.006 共1.007兲
Tx
共Dt兲Rx
具D共r៝兲典xRx
具D共r៝兲典xref
Prescription parameters
kkD,yyS assumed by DRx
t
Tx
TABLE V. Ratios of administered-to-prescribed dose, 共Dt兲Rx
, as a function of year t and dosimetry data tD for
125
I implants. Bold indicates current analysis, using GFSA method.
DxTx
共Gy兲
DxRx
共Gy兲
Williamson et al.: Dose specification for
a
1438
125
I interstitial brachytherapy
1438
these source models were relatively new products, any dosedelivery errors associated with their use are irrelevant to the
interpretation and duplication of the published and evaluated
clinical experience. As discussed in more detail in Sec. III C
Model 6711 125I vendor calibrations were not affected by the
1999 SK,N99 measurement anomaly.
Three different methods for evaluating the impact of dosimetry changes on prostate implants were employed. The
RDA method is the simplest and the most clear intuitively in
that the delivered doses will scale proportionally with the
product of dose rate constant and the anisotropy constant
provided the radial dose function is unchanged. Like the
RDA method, the GFSA is also a single source method and
uses inverse square-law 共approximately兲 to weight the dose
contributions at different distances. The most comprehensive
method is the CIA method, which uses typical implant geometries for averaging the dose contributions from different
distances and from many different sources. The next level
calculational techniques, which can further enhance the
evaluation of dosimetric impact, would be to use theoretical
models of radiation induced cell killing 共for example, Yue et
al.17兲. At this stage we feel that the CIA model is quite adequate for the analysis presented considering the lack of sensitivity of these calculation results to the method used. Even
the simplest RDA method using the product of dose rate
constant and anisotropy constant gives results within 2% of
the results from CIA method. Thus, the analysis presented
here is relatively insensitive to the choice of calculational
technique.
Our analysis indicates that the full implementation of
2004 TG-43 report recommendations will have no significant
impact on the 125I dose prescriptions and dose delivered 共less
than 2%兲. However, for 103Pd sources, there will be a systematic escalation of dose delivered by about 4.2% compared
to current practice unless the prescribed doses are revised
downward from 125 Gy. The radiation oncologist has three
choices: 共1兲 stay with the current dose prescription of 125 Gy
and accept a 4% dose escalation; 共2兲 decrease the prescribed
dose to a round number of 120 Gy which will deliver doses
very close to the current practice; or 共3兲 decrease the prescribed dose to 115 Gy, which will restore future delivered
doses to levels characteristic of pre-1997 delivery practices.
Of course, radiation oncologists should also consider their
own clinical techniques and experiences in terms of disease
control and toxicity, which depend upon many procedural
and technical details such as the choice of margins around
the tumor volume, the loading pattern, number of needles
used and patient selection for brachytherapy. For the sake of
consistency at the national level, the AAPM recommends
that the radiation oncology physician community review this
report and make recommendations regarding the need for
prescribed dose revision, if any, similar to the American
Brachytherapy Society recommendations5 published in response to the AAPM 2000 guidance.1
Although we have focused on the dose prescription for
prostate cancer only, the methods described here are applicable for implants at any other site. However, caution should
ជ兲 / DRx
ជ兲典 ratios to
be exercised in extrapolating the 具Dref
t 共r
t 共r
1439
Williamson et al.: Dose specification for
103
Pd and
125
I interstitial brachytherapy
implants that differ significantly from the geometry of typical
prostate volume implants, e.g., planar implants or eye
plaques. In such cases, it may be prudent to re-evaluate the
ជ兲 / DRx
ជ兲典 ratio for the specific implant geometry in
具Dref
t 共r
t 共r
question.
Finally, it should be emphasized that the impact of the
adoption of the 2004 AAPM TG-43 report recommendations
is a systematic change that would affect all patients under
treatment if adopted uniformly. Therefore, such systematic
changes, even though they may appear small for an individual patient, especially considering the dose inhomogeneity within a tumor volume, can have a profound effect on the
efficacy of a treatment regimen. Thus, a careful consideration
of all clinical factors is necessary before making systematic
changes in dose prescription and dose delivered by a
thoughtless adoption of a new dosimetry protocol.
WHOM TO CONTACT FOR FURTHER ASSISTANCE
If you have questions regarding the recommendations of
this report or implementing them in your clinic, please contact the Radiological Physics Center 共RPC兲 at MD Anderson
Cancer Center, Houston, TX at 共713兲 792-3226.
ACKNOWLEDGMENTS
The authors would like to thank Dr. Ty Robin, Dr. Mary
Napolitano, and Joe Rodgers of Theragenics Corporation for
their exceptionally detailed and helpful comments.
Certain commercial equipment, instruments, or materials
are identified in this paper to foster understanding. Such
identification does not imply recommendation or endorsement by the AAPM or the National Institute of Standards and
Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
a兲
Electronic mail: jwilliamson@mcvh-vcu.edu
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S. M. Seltzer et al., “New national air-kerma-strength standards for 125I
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