Monte Carlo dosimetry for125I and103Pd eye plaque brachytherapy
with various seed models
R. M. Thomsona?and D. W. O. Rogers
Ottawa Carleton Institute for Physics, Carleton University Campus, Ottawa, Ontario K1S 5B6, Canada
?Received 4 August 2009; revised 23 October 2009; accepted for publication 10 November 2009;
published 23 December 2009?
Purpose: Dose distributions are calculated for various models of
standardized plaques of the Collaborative Ocular Melanoma Study ?COMS?. The sensitivity to seed
model of dose distributions and dose distributions relative to TG-43 are investigated.
Methods: Monte Carlo simulations are carried out with the EGSnrc user-code BrachyDose.
Brachytherapy seeds and eye plaques are fully modeled. Simulations of one seed in the central slot
of a 20 mm Modulay ?gold alloy? plaque backing with and without the Silastic ?silicone polymer?
insert and of a 16 mm fully loaded Modulay/Silastic plaque are performed. Dose distributions are
compared to those calculated under TG-43 assumptions, i.e., ignoring the effects of the plaque
backing and insert and interseed attenuation. Three-dimensional dose distributions for different125I
and103Pd seed models are compared via depth-dose curves, isodose contours, and tabulation of
doses at points of interest in the eye. Results are compared to those of our recent BrachyDose study
for COMS plaques containing model 6711 ?125I? or 200 ?103Pd? seeds ?R. M. Thomson et al., Med.
Phys. 35, 5530–5543 ?2008??.
Results: Along the central axis of a plaque containing one seed, variations of less than 1% are seen
in the effect of the Modulay backing alone for different seed models; for the Modulay/Silastic
combination, variations are 2%. For a 16 mm plaque fully loaded with125I ?103Pd? seeds, dose
decreases relative to TG-43 doses are 11%–12% ?19%–20%? and 14%–15% ?20%? at distances of
0.5 and 1 cm from the inner sclera along the plaque’s central axis, respectively. For the same
prescription dose, doses at points of interest vary by up to 8% with seed model. Doses to critical
normal structures are lower for all103Pd seed models than for125I with the possible exception of the
sclera adjacent to the plaque; scleral doses vary with seed model and are not always higher for
103Pd than for125I.
Conclusions: Dose decreases relative to doses calculated under TG-43 assumptions vary slightly
with seed model ?for each radionuclide?. Dose distributions are sensitive to seed model; however,
variations are generally no larger than the magnitudes of other systematic uncertainties in eye
plaque therapy. © 2010 American Association of Physicists in Medicine.
103Pd seeds in the
Key words: eye plaque, COMS, choroidal melanoma, BrachyDose, Monte Carlo, dose calculation,
Choroidal melanoma is the most common primary intraocu-
lar malignancy. Therapeutic interventions for this cancer
were evaluated in the Collaborative Ocular Melanoma Study
?COMS?, initiated in the mid-1980s. In 2001, the COMS
group reported that survival rates for enucleation ?removal of
the eye? and125I eye plaque brachytherapy were the same,1
and this was later confirmed in 2006.2Plaques of a standard-
ized design3were used for COMS, consisting of a Modulay
?gold alloy? backing containing a Silastic ?silicone polymer?
seed carrier. COMS plaques continue to be widely used for
eye plaque therapy.
The protocol described by Task Group 43 ?TG-43? of the
American Association of Physicists in Medicine ?AAPM?4,5
is widely used for treatment planning for eye plaque
therapy.6In this approach, inhomogeneities in the seeds’ vi-
cinity ?plaque backing, insert, air, etc.? are ignored and dose
to water is calculated. Taking into account the Modulay
backing and Silastic insert and seed-to-seed interactions,
dose decreases relative to TG-43 calculations are 14% for
125I ?Oncura/GE/Amersham Health Oncoseed 6711? and
20% for103Pd ?Theragenics TheraSeed 200? at 1 cm from the
inner sclera on the central axis of COMS plaques.7,8Doses to
critical normal structures in the eye can differ by 30% or
more compared to TG-43 calculations.7Dose reductions of
up to 10% result from replacing a portion of the water phan-
tom with air.7,9Delivered doses thus differ significantly from
those calculated under the TG-43 protocol.
Recent Monte Carlo studies7,8of dosimetry for eye plaque
therapy have focused on model 6711 and 200 seeds for125I
and103Pd, respectively; however, other seed models are used
for eye plaque therapy. In addition to these two seed models,
six other seeds with adequate air kerma strength for eye
plaque therapy have been identified ?Table I?. Seed models
differ through the shape and thickness of encapsulation, the
368368Med. Phys. 37 „1…, January 20100094-2405/2010/37„1…/368/9/$30.00© 2010 Am. Assoc. Phys. Med.
distribution of radioactive material within the encapsulation,
the type of radiographic marker, and materials.10Physical
characteristics of photons emitted by different seed models
?position, energy, location, and direction? are strongly depen-
dent on seed construction, especially since brachytherapy
sources are often constructed of high atomic number
materials.11These characteristics affect interactions in the
plaque’s Modulay backing and Silastic insert,7as well as
seed-to-seed interactions.12Thus, differences in seed models
will be reflected in dose distributions for eye plaque therapy.
In this work, the sensitivity of dose distributions to seed
model for eye plaque therapy using COMS plaques is studied
using Monte Carlo methods. The seed models considered are
those listed in Table I. Given the state of flux of seed models
on the market, this is likely not an exhaustive list of seeds
which may be used for eye plaque therapy; however, it
should be adequate to illustrate differences between seed
models. The effects of the Modulay backing alone and the
Modulay backing and Silastic insert combination on doses
are compared for a single seed of each model. Three-
dimensional dose distributions for a fully loaded COMS
16 mm plaque are calculated in the eye region for each seed
model. Dose distributions are compared to those calculated
under TG-43 assumptions to ascertain whether differences
relative to TG-43 vary with seed model. Dose distributions
are compared via depth-dose curves, isodose contours, and
tabulation of doses at points of interest.
Simulations are performed with the EGSnrc ?Refs. 13 and
14? user-code BrachyDose.15,16Simulation details ?e.g., dose
scoring, physics processes simulated, and plaque descrip-
tions including composition of Modulay and Silastic? are de-
scribed in our BrachyDose study of COMS plaques contain-
ing models 6711 ?125I? and 200 ?103Pd? seeds.7As such,
many details are omitted from this section.
The COMS 20 mm plaque with a single seed in its central
slot and the COMS 16 mm plaque fully loaded with 13 seeds
are both simulated. Plaques are modeled at the center of a
?30 cm?3water phantom of mass density 0.998 g/cm3; 1011
histories are simulated and dose is scored in ?0.05 cm?3vox-
els. Simulations of the fully loaded 16 mm plaque are com-
pared to simulations performed under TG-43 assumptions,
herein referred to as “TG-43-sim,” with the same configura-
tion of seeds in water and no interseed interactions, plaque
backing, or insert.7,17
The seed models used in this study ?Table I? were previ-
ously benchmarked18,19and dosimetry parameters for each
seed model appear in the Carleton Laboratory for Radio-
therapy Physics TG-43 Database.20Table I lists the average
energy of photons emitted from seeds of each model, as re-
ported by Usher-Moga et al.,10based on measurements of the
seeds’ spectroscopic output with a high-purity germanium
detector. Due to the fluorescence x rays from silver ?22,
25 keV?, the average energy of photons emitted from
seed models containing silver ?6711, 3500, and IAI-125? is
slightly lower than for seed models without silver ?2301 and
1251L?. The average energy of photons emitted ?above 5
keV? for a bare
In Table I, the extent of radioactivity in the brachytherapy
sources is the distance along a seed’s longitudinal axis be-
tween the two extremities of radioactive material. For ex-
ample, the extent of radioactivity is the cylinder length for
cylindrical sources and the distance between the outer edges
of the outermost beads for bead sources. In some cases, this
length coincides with the “active length” defined by
The eye ?X,Y,Z? and plaque ?x,y,z? coordinate frames
were described previously.7,17The origin of the eye reference
frame ?X,Y,Z? is at the center of the eye ?Fig. 1?. The eye is
idealized as a sphere of diameter 2.46 cm with a sclera
0.1 cm thick. The plaque reference frame ?x,y,z? has its
origin at the inner sclera ?0.1 cm inset from the concave side
of the Silastic insert? on the plaque’s central axis. The z axis
coincides with the plaque’s central axis.
Doses are tabulated at eight points of interest in the eye
region ?Fig. 1? for a right eye: sclera adjacent to the plaque,
125I ?103Pd? source is 28.37 keV ?20.74
TABLE I. Brachytherapy sources simulated. Trade names and the average energy of photons emitted from seeds
are those quoted by Usher-Moga et al. ?Ref. 10?; the average energy is not available for the model IAPd-103
seed. The extent of ?radio?activity ?distance along a seed’s longitudinal axis between the two extremities of
radioactive material? is estimated using the descriptions of Taylor and Rogers ?Refs. 19 and 20?.
RadioisotopeManufacturer Model Trade name
Best Medical International
International Brachytherapy Co.
Implant Sciences Corporation
Best I-125 Source
Best Medical International
Best Pd-103 Source
369 R. M. Thomson and D. W. O. Rogers: Eye plaque brachytherapy dosimetry with different seed models 369
Medical Physics, Vol. 37, No. 1, January 2010
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6COMS Group, Ch 12: Radiation Therapy, COMS Manual of Procedures
PB95–179693, Springfield, VA, National Technical Information Service
?1995?, 1999Revisionat http://www.jhu.edu/wctb/coms/manual/
coms_chap12.pdf, accessed October 15, 2009.
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teristics,” Med. Phys. 23, 407–418 ?1996?.
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Beaulieu, “A Monte Carlo study on the effect of seed design on the
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22S. Chiu-Tsao, personal communication ?2009?.
103Pd iris melanoma
Parameter Database, http://
376R. M. Thomson and D. W. O. Rogers: Eye plaque brachytherapy dosimetry with different seed models376
Medical Physics, Vol. 37, No. 1, January 2010