ROPES eye plaque brachytherapy dosimetry for two models of 103Pd seeds

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Australasian Physical &Engineering Sciences inMedicineThe Official Journal of theAustralasian College ofPhysical Scientists andEngineers in Medicine ISSN 0158-9938Volume 34Number 2 Australas Phys Eng Sci Med(2011) 34:223-231DOI 10.1007/s13246-011-0069-1

ROPES eye plaque brachytherapydosimetry for two models of 103Pd seeds

Pooneh Saidi, Mahdi Sadeghi, AlirezaShirazi & Claudio Tenreiro

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SCIENTIFIC PAPER

ROPES eye plaque brachytherapy dosimetry for two modelsof 103Pd seeds

Pooneh Saidi • Mahdi Sadeghi • Alireza Shirazi •

Claudio Tenreiro

Received: 30 October 2010 / Accepted: 18 March 2011 / Published online: 31 March 2011

� Australasian College of Physical Scientists and Engineers in Medicine 2011

Abstract Brachytherapy dose distributions are calculated

for 15 mm ROPES eye plaque loaded with model Thera-

genics200 and IR06-103Pd seeds. The effects of stainless

steel backing and Acrylic insert on dose distribution along

the central axis of the eye plaque and at critical ocular

structure are investigated. Monte Carlo simulation was

carried out with the Version 5 of the MCNP. The dose at

critical ocular structure by considering the eye composition

was calculated. Results are compared with the calculated

data for COMS eye plaque loaded with Theragenics200

palladium-103 seeds and model 6711 iodine-125 seed. The

air kerma strength of the IR06-103Pd seed to deliver 85 Gy in

apex of tumor in water medium was calculated to be 4.10

U/seed. Along the central axis of stainless steel plaque loa-

ded with new 103Pd seeds in Acrylic insert, the dose reduc-

tion relative to water is 6.9% at 5 mm (apex). Removal of the

Acrylic insert from the plaque (replacing with water) did not

make significantly difference in dose reduction results

(*0.2%). The presence of the stainless steel backing results

in dose enhancement near the plaque relative to water. Doses

at points of interest are higher for ROPES eye plaque when

compared to COMS eye plaque. The dosimetric parameters

calculated in this work for the new palladium seed, showed

that in dosimetry point of view, the IR06-103Pd seed is

suitable for use in brachytherapy. The effect of Acrylic insert

on dose distribution is negligible and the main effect on dose

reduction is due to the presence of stainless steel plaque

backing.

Keywords 103Pd � Eye plaque � Brachytherapy �Dosimetry

Introduction

Choroidal melanoma is the most common primary intra-

ocular malignancy in adults, originating within the pig-

mented cells of the choroid [1]. Management of patients

with this neoplasm is complex and remains the subject of

much discussion [2]. The treatment modality of choice is

predicated in part by the size and location of the tumor [3].

Patients with a clinical diagnosis of medium-sized cho-

roidal melanoma (between 2.5 and 10 mm in height and

\16 mm basal diameter) are candidates for episcleral

plaques if the patient is otherwise healthy and without

metastatic disease [4]. Plaque brachytherapy is currently

the most common treatment option for these patients. It

offers good chances of conserving the eye, often with at

least some useful vision [5, 6]. The procedure is relatively

easy and safe to perform on almost any patient and the

costs are reasonable. Compared to charged particle radia-

tion the main disadvantage of brachytherapy is the less

sharply defined intraocular dose distribution, which limits

the ability to shield healthy ocular tissues from radiation.

Several types of eye plaque are used for treatment of

P. Saidi

Department of Nuclear Engineering, Research and Science

Branch, Islamic Azad University, Tehran, Iran

M. Sadeghi (&)

Agricultural, Medical & Industrial Research School,

Nuclear Science and Technology Research Institute,

Karaj, P.O. Box 31485/498, Tehran, Iran

e-mail: [email protected]

A. Shirazi

Department of Biophysics, Faculty of Medicine,

Tehran University of Medical Sciences, Tehran, Iran

C. Tenreiro

Faculty of Engineering, Talca University, Talca, Chile

123

Australas Phys Eng Sci Med (2011) 34:223–231

DOI 10.1007/s13246-011-0069-1

Author's personal copy

intraocular tumors, which are most often rounded, made of

gold or stainless steel (iodine or palladium) and come in

several diameters in order to effectively treat tumors of

different sizes. Iodine-125 is currently the most commonly

used, and well documented in the literature [7–11]. Few

centers use palladium-103, but available reports indicate

that because of its low energy emissions 21 keV which

allow for a rapid decrease in dose with distance; and also,

the short half-life, 17 days, result in higher dose rates and

favorable dose distribution [12–15]. Due to the low energy

of the photons from 103Pd the effect of backscatter from

plaque backing on dose distribution should be significant.

Many reports are available concerning to the effect of the

plaque backing on dose rate mostly based on gold backing.

Cygler et al. [16], Wu et al. [17] and Luxton et al. [18] by

experimental measurement, and Thomson et al. by Monte

Carlo simulation [19], obtained the effect of the gold

backing on the dose rate around the 125I seed. They

reported a dose enhancement near the seed due to the

backscatter photons from the gold backing. But Chiu-Tsao

et al. [20], Weaver [21] and de la Zerda et al. [22] based on

their experimental measurements (TLD), reported that dose

at small distances from the seed was reduced due to pres-

ence of gold backing. As quoted above there are conflicted

results in the publications and also most of the publications

are limited to study the effect of the gold backing (COMS

plaque) on dose rate near the 125I seeds. While the

dosimetry studies for COMS (collaborative ocular mela-

noma study [6]) eye plaque exist and these plaques are

widely used, ROPES (radiation oncology physics and

engineering services, Australia) plaques are also used and

dosimetry has not been thoroughly studied. The ROPES

eye plaque has been studied for I-125 by Granero et al.

[23]. Thus, in this work the Monte Carlo technique is used

to make a study of dose rate distributions around the

15 mm ROPES eye plaque fully loaded with two palladium

seed models, Theragenics200 and IR06-103Pd. The latter is

a new palladium brachytherapy seed, which has been

developed by Agricultural, Medical & Industrial Research

School. According to American Association of Physicists

in Medicine, (AAPM) TG-43U1 recommendations, before

using each new source clinically, the dosimetric charac-

teristics of the source must be determined to provide reli-

able data for use in treatment planning calculations and

dose prescription [24]. So according to TG-43U1 guide-

lines the dosimetric parameters of the new palladium

source were calculated. A comparison of the derived

IR06-103Pd seed dosimetry calculated data with data for

other commercial palladium seeds is presented. Since the

tumor control rate for plaque brachytherapy is high, the

most important problem is side effects of the healthy

structures (points of interest) in eye region. Since the points

of interest are as close as few millimeters from the source,

the received dose by these points should be considered.

Other Monte Carlo simulations have been employed to

investigate the effect of the plaque backing and acrylic

insert on dose distributions, in the following conditions:

1. Acrylic insert with stainless steel backing;

2. Acrylic insert with water replacing stainless steel

backing;

3. Liquid water replacing acrylic, with stainless steel;

4. Liquid water replacing acrylic and stainless steel

(seeds alone in water).

Finally Doses along the plaque’s central axis and at the

critical points are compared for 103PdTheragenics200 and

IR06-103Pd seeds loaded in ROPES eye plaque with 125I

(6711 model) and 103Pd (Theragenics200 model) seeds

loaded in COMS eye plaque [19, 25, 26].

Materials and methods

Dose distributions in this work were simulated with the

MCNP5 Monte Carlo (MC) radiation transport code pub-

lished by Los Alamos National Laboratory and the MCP-

LIB04 photon cross-section library is based on the ENDF/

B-VI data [27]. The 103Pd photon spectrum used in these

simulations was obtained from TG-43U1 Table XIII [24].

Particle fluence and cell-heating tallies, F4 and F6,

respectively were employed to calculate kerma and

absorbed dose in this study.

103Pd source description

Figure 1, shows a schematic diagram of the IR06-103Pd

seed. The seed contains five resin beads, each in diameter

of 0.6 mm with the compositions of (by weight percent):

H, 8%; C, 90%; N, 0.3%; Cl, 0.7%; and Pd, 1%; and the

density equal to 1.14 g/cm3, which are packed inside a

titanium cylinder of 4.7 mm length, 0.7 and 0.8 mm

internal and external diameters, respectively, 0.6 mm thick

end caps and with an effective length of 3 mm. 103Pd

radioactive material is absorbed uniformly in the resin bead

volume.

Fig. 1 Schematic diagram of the IR06-103Pd seed

224 Australas Phys Eng Sci Med (2011) 34:223–231

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Seed Monte Carlo dosimetry

The IR06-103Pd source has been simulated in the center of

a spherical phantom of water with 15 cm radius with an

array of 1 mm thick detector rings. The rings were bounded

with two cones (10�) bisecting the sphere corresponding to

points in two-dimensional TG-43U1 dose formalism [24].

Detectors were defined at distances of r = 0.25, 0.5, 0.75,

1, 2, 3, 4, 5 and 7 cm, away from the source and at polar

angles relative to the seed longitudinal axis from 0� to 90.

Due to the low energy of the photons from 103Pd, it was

assumed in the Monte Carlo calculations all electrons

generated by the photon collisions are absorbed locally, so

dose is equal to kerma at all points of interest [28]. The SK

of the IR06-103Pd seed was estimated by calculating the

dose in 1 mm-thick air-filled rings in a vacuum. The rings

were bounded by 86� and 94� conics and defined with a

radial increment of 5 cm to 150 cm along the transverse

axis of the source, and the value of SK was found to be

independent of distance [29]. In these calculations, the

titanium characteristic X-ray production were suppressed

with d = 5 keV (d is the energy cutoff) [30]. The MCNP

F6 tally was used to calculate the dose distribution around

the seed.

According to the TG-43 report, the role of the geometry

factor, G(r, h), is to suppress the influence of the inverse

square law on the radial dose function and the anisotropy

function and it provides a definition of the geometry factor

in two simple forms, one for point sources and one for line

sources [31].

g(r) was calculated by using line (with an effective

length of 3 mm) and point-source geometry for IR06-103Pd

seed. In order to obtain the dose rate distributions, the

source was positioned at the center of a 30 cm diameter

spherical phantom with an array of 1 mm thick detector

rings. The rings were bounded with two cones (10�)

bisecting the sphere corresponding to points in two-

dimensional TG-43U1 dose formalism [24].

The simulations were performed up to 1 9 109 histories

in water with statistical uncertainties of 0.05% to 0.1% at 1

and 5 cm on the transverse plane and 2.5% and 3.5% at 1

and 5 cm along the long axis. In air with 7 9 107 histories,

statistical uncertainty was 1%. With this number of histo-

ries, dosimetry results have a 1d B 2% (k = 1, 67% con-

fidence index) at r B 5 cm and also air kerma rate

calculations for derivation of air kerma strength, have

1d B 1% at the point of interest.

The MCNP simulation method in this work was

benchmarked with brachytherapy seed model Theragenics

200 [24, 25].

For the benchmarking process, the dosimetric parame-

ters of seed model Theragenics 200 were obtained by our

simulation method and the results are compared by the

published data for the seed, to verify our simulation

method.

ROPES plaque simulation

The ROPES plaque with diameter of 15 mm is composed

of two parts:

1. The stainless steel backing with the compositions of

(by weight): (C, .0003); (Si, .01); (Mn, .02); (P, .0004);

(S, .0003); (Cr, .17); (Mo, .025); (Ni, .12) and (Fe,

.654); with a density of 7.912 g/cm3. The eye plaque is

consisted of 1 mm thick stainless steel backing with

inner and outer radius of curvature of 13 mm and

14.6 mm, respectively and 3 mm height for the

collimating lip (shell) [23].

2. The Acrylic insert as a seed career with composition

by formula of (H, 8); (C, 5) and (O, 2) and density of

1.069 g/cm3. The thickness of the seed career is 2 mm

and 103Pd seeds are loaded in ten slots arranged in two

concentric rings in about the plaque’s central axis. The

inner and outer rings consist 3 and 7 seeds in 2p/3 and

2p/7 rotation angles respectively [23]. In this study the

Rayleigh scattering, Compton scattering, photoelectric

absorption and fluorescent emission of characteristic

K-shell and L-shell X-rays are all modeled. The effect

of plaque backing and polymeric insert on dose

distribution in the eye region is also investigated.

The plaque assumed a standard eye diameter of 24.6 mm

[19] by considering lens and homogenized eye materials

according to ICRU 46 as an eye model as quoted in Ref. [19].

The position of the points were taken from Thomson et al.’s

manuscripts (Fig. 2), [19, 26]. Eye ball and eye plaque

modeled at the center of 30 cm spherical water phantom.

The effect of plaque backing and Acrylic insert on dose rate

is provide with water replacing by stainless steel and Acrylic

insert. Central axis depth dose were calculated by F6 tally

using 0.05 mm-radius and 0.01 mm thick cylindrical voxels

from outer sclera (-1 mm) to 11 mm inside the eye in

0.5 mm steps. For the comparison the doses at the interest

points in eye such as center of eye, macula, optic disk, sclera,

apex, lacrima gland and opposite side, with other published

data, the dose is calculated as follows: ‘‘The Monte Carlo

simulations provide the dose in a voxel per history. The dose

rate is calculated by dividing this number by the air kerma

strength per history for the relevant seed type and multi-

plying by the number of seeds and the air kerma strength per

seed. The air kerma strength per seed is chosen in order to

obtain a prescription dose of 85 Gy at the tumor apex (5 mm

on the central axis) in 168 h for 103Pd. The total dose

delivered during a treatment is then determined by inte-

grating over the treatment time, taking into account the

exponential decay of the source’’. 103Pd with dose rate of

Australas Phys Eng Sci Med (2011) 34:223–231 225

123


0.5 Gy/h deliver the prescription dose of 85 Gy in 168 h [4,

19, 28].

In general, up to 3.8 9 108 photon histories were sim-

ulated and the statistical uncertainties of 0.7% and 1.1%

were obtained at 5 mm and 11 mm (tumor apex) depth of

central axis respectively. Statistical uncertainties exceeded

4% at the opposite side of the eye which has the largest

uncertainty among the interest points. According to the

calculated uncertainties, the dosimetry results have a 1d(k = 1) B 2% at r B 5 cm. which is covered the ‘‘Good

practice for Monte Carlo calculations’’ recommended in

TG-43U1 protocol [24].

Results and discussion

Seed dosimetry benchmarking

Table 1 shows the calculated dose rate constant, K, for the

IR06-103Pd seed and the calculated and measured values of K,

for the model MED3633, Theragenics model 200 and Best

double-wall sources. Based on the calculations, the dose rate

constant for the IR06-103Pd source estimated to be

0.692 ± 0.020 cGy U-1 h-1 which is comparable with

the two other commercial sources. The comparison of calcu-

lated value of K for the model 200 in this study,

0.685 cGy h-1 U-1, with the previously published data for

the seed, [24] 0.686 cGy h-1U-1, (*0.1% difference) dem-

onstrate the accuracy of our simulation method (Table 1).

Calculated Air-kerma strength, per contained activity for

model 200 in this work was 0.722 U mCi-1. The result was

compared to 0.721 U mCi-1 (0.14%) for Williamson’s

WAFAC simulation [25], and Melhus et al. [29] calculations.

The calculated line radial dose function gL(r), of the

IR06-103Pd, was fit to a fifth order polynomial function

yielded the following relationship and coefficients:

gL rð Þ ¼ a0 þ a1r þ a2r2 þ a3r3 þ a4r4 þ a5r5;

where a0 = 1.785, a1 = -1.064 cm-1, a2 = 3.385 9

10-1 cm-2, a3 = -7.062 9 10-2 cm-3, a4 = 8.469 9

10-3 cm-4 and a5 = -4.173 9 10-4 cm-5, define

R2 = 9.999 9 10-1.

The radial dose function, g(r), for IR06-103Pd seed and

three other commercial sources is presented in Fig. 3. The

differences between the results of this study and AAPM

TG-43U1 reference data for the model 200 were presented

in Table 2. The agreement between these data sets was

acceptable and the differences for radial dose function and

dose rate constant were less than 3% and 1%, respectively.

Figure 4 shows a comparison between the Monte Carlo

calculated anisotropy function of the IR06-103Pd seeds at

distance of 2 cm from the source center in water with the

published data. The values of calculated anisotropy func-

tion for the new 103Pd sources agreed with those for the

model MED3633, Theragenics model 200 and Best�

double-wall 103Pd sources [24, 32] within 4% in angles

greater than 20�. The differences in smaller angles can be

as large as up to 17%, due to thicker end caps of

IR06-103Pd source in compare with three other sources.

Plaque Monte Carlo simulations

Table 3 presents the calculated central depth dose distri-

bution in 0.5 mm steps between the outer sclera (-1 mm)

to 3 mm and in 1 mm steps from 4 mm to 10 mm for

ROPES eye plaque loaded with each model Theragen-

ics200 and IR06-103Pd seeds in the Acrylic insert in com-

pare with the same data for COMS eye plaque loaded with

model Theragenics200 seeds [29] the results were com-

paring to the seeds alone in water. In this study, the

required air-kerma strength per seed (SK) to deliver pre-

scription dose (85 Gy) to the apex of tumor (5 mm depth)

in water medium for 168 h implant is obtained equal to

4.1 U/seed and 4.11 U/seed for IR06-103Pd and model

Theragenics200, respectively. In addition, to investigate

the effect of different materials constituting the ROPES

plaque on dose distributions near the plaque, the ratio of

dose in three medium (discussed above) relative to dose in

water medium is shown in Fig. 5.

Effect of stainless steel backing

The effect of stainless steel plaque backing on dose dis-

tribution along the central axis is shown in Fig. 5, which

Fig. 2 Points of interest for eye plaque dosimetry, given in the center

of eye reference frame (scale in centimeters) for a right eye [19].

15 mm ROPES eye plaque positioned in four position for the

simulation: two positions when plaques centered on the equator

(temporal and nasal), and two positions in the midway between the

equator and posterior pole (inferior and superior)


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provides central axis depth dose curve for fully loaded

IR06-103Pd seeds with water replacing the Acrylic insert.

This figure demonstrates due to fluorescence photons

emitted by atoms in the plaque backing, total dose is

increased near the plaque. Emission photons from palla-

dium seeds with average energy of about 21 keV, excite

the K-shell in iron (*7 keV) [33] in chromium (*6 keV),

in molybdenum (*20 keV), and in manganese

(*6.5 keV) [34] which are the composition of the plaque

backing. The excitation of these shells results in the

emission of fluorescence photons, so this event explains

why dose increases near the plaque. About 1% dose

enhancement is observed near the plaque but after a few

millimeters the dose decreases because the fluorescence

photons are absorbed (mean free path is about 2 mm) and

the dose decrease at prescription point (5 mm) and oppo-

site side of the eye is of the order of 6.8–7.1%. Thomson

et al. [19] reported a dose decrease of about 6–6.3% at

opposite side of the eye for 103Pd (Theragenics model 200)

seed in gold backing. Simulation by Chiu-Tsao et al. [20]

yielded a dose decrease of about 10% for 125I (model 6711)

seed with 20 mm gold plaque only at 7.6 mm. the emitted

photons from the 125I seed have higher energy than those

Table 1 Monte Carlo calculated dose rate constant, K, of the IR06-103Pd seed and model Theragenics200 source and comparison with the

measured and calculated values of model MED3633, Theragenics200 and Best sources

Source type Method Medium K (cGy h-1 U-1)

IR06-103Pd Monte Carlo simulationa Liquid water 0.692 ± 0.02

MED3633 TLD dosimetryb Solid water 0.688 ± 0.05

Monte Carlo simulationc Liquid water 0.677 ± 0.02

Theragenics 200 TLD dosimetryd Solid water 0.650 ± 0.08

Monte Carlo simulatione Liquid water 0.686 ± 0.03

Monte Carlo simulationa Liquid water 0.685 ± 0.02

Best103Pd TLD dosimetryf Solid water 0.69 ± 0.08

Monte Carlo simulationf Liquid water 0.67 ± 0.02

a Present work, b Reference [35], c Reference [36], d Reference [37], e Reference [25], f Reference [32]

Fig. 3 Comparison of calculated radial dose function of IR06-103Pd

seeds in water versus three other available sources [24, 32]

Table 2 Monte Carlo calculations for radial dose function, gL(r), for

new palladium seed (IR06-103Pd) and for model Theragenics 200

source in comparison with reference Monte Carlo data

gL(r)

r (cm) Theragenics200 Percent-

difference

IR06-103Pd

Reference

[24]

Present

work

0.5 1.300 1.330 -2 1.333

0.75 1.150 1.170 -2 1.144

1 1.000 1.000 0 1.000

1.5 0.749 0.755 -1 0.756

2 0.555 0.567 -2 0.566

3 0.302 0.305 -1 0.318

4 0.163 0.168 -3 0.168

5 0.089 0.091 -3 0.091

7 0.026 0.027 -2 0.026

Fig. 4 Comparison of the calculated anisotropy function of the

IR06-103Pd seed versus three other sources at 2 cm [24, 32]


123


emitted by 103Pd seed so more fluorescence photons is

observed when 125I source is used.

Due to the emission of fluorescence photons from the

plaque backing for all seed and backing models without

any polymer insert, there is a small dose enhancement near

the plaque. The spectrum of fluorescence photons depends

on the energy of photons emitted by brachytherapy sources,

its active length and also depends on the composition of

plaque backing.

Effect of acrylic insert

The central axis doses for ten IR06-103Pd seeds in acrylic

insert with plaque backing replaced with water are shown

in Fig. 5 relative to the doses for the same seeds in water

medium.

Acrylic with an effective atomic number of (*6.5) has

attenuating effect as similar as water with an effective atomic

number of Zeff * 7.4 [19]. The average variation in dose

distribution due to acrylic insert (without plaque) related to

water is about 0.2%, so the presence of acrylic insert can be

negligible in the calculations. Granero et al. [23] have

obtained similar result with ROPES plaque loaded by 125I

model 6711 seeds in Acrylic insert. Thomson et al. [19]

reported 17% dose reduction for model 200 103Pd seed at

distance of 1 cm in COMS plaque due to presence of Silastic

insert, with an effective atomic number of 10.7 which is more

attenuating medium than water. Chiu-Tsao et al. [20]

obtained 10% dose reduction at 1 cm for Silastic insert only,

in 20 mm COMS eye plaque for 125I and 16% for 103Pd

(without gold backing) relative to water along the central

axis; Also they claimed the effect of gold/Silastic combi-

nation is comparable with the effect of Silastic insert only.

The dose reduction of 125I source in Silastic insert is more

than 103Pd source due to its higher energy of emitted photons.

Table 3 Central axis dose distributions for 15 mm ROPES eye

plaque loaded with ten IR06-103Pd and model Theragenics200 seeds

are compared with doses of COMS eye plaque loaded with

Theragenics200. The ratio of doses in central axis with Acrylic

insert ? Stainless steel backing to the doses in water medium for two

palladium seed models in ROPES plaque, is also included

Central axis depth (mm) 15 mm ROPES eye plaquea 16 mm COMS eye plaqueb

IR06-103Pd Theragenics200 Theragenics200

Water medium Stainless steel ? Acrylic Water medium Stainless steel ? Acrylic Gold ? Silastic

-1 308.81 311.88 308.01 310.9 279.0

-0.5 305.49 305.00 305.43 305.47 277.7

0 271.49 268.67 277.53 287.01 259.1

0.5 266.97 265.79 257.31 261.62 234.2

1 243.94 234.18 245.09 241.44 210.4

1.5 214.37 201.50 211.26 208.01 189.1

2 188.89 173.77 195.74 186.0 169.0

2.5 164.33 149.54 181.28 166.23 151.3

3 148.18 133.36 159.79 148.51 134.1

4 120.35 108.31 131.96 118.52 107.2

5 (Apex) 85.00 79.15 85.00 73.0 85.0

6 84.53 75.65 84.65 75.61 68.1

7 68.99 61.40 68.11 60.73 54.3

8 55.35 48.70 55.27 48.51 44.1

9 40.53 35.66 41.39 39.71 36.1

10 28.84 25.72 30.74 32.34 29.4

SK 4.10 U/seed 4.11 U/seed

a Present work, b Reference [29]

Fig. 5 Ratio of the doses along the plaque’s central axis for 15 mm

ROPES plaque fully loaded with IR06-103Pd to the doses in water

medium


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The dose reduction for the stainless steel/acrylic combina-

tion relative to water medium as shown in Fig. 5 is about

12% at 1 cm and 11% and at 0.5 cm. Simulation of ROPES

eye plaque for 125I by Granero et al. [23] demonstrated 4%

dose decrease. Because of lower energy photons of 103Pd and

the shorter mean free path of them than 125I, the more dose

reduction is observed. Thomson et al. have obtained a

reduction of the dose relative to water of 20% for 103Pd seed

at 1 cm in the COMS plaque central axis, the main reduction

is due to the Silastic insert.

Dose comparison at points of interest

To calculate the dose to critical structures in the eye region,

eye composition was considered in the simulation and

simulations were done for few different plaque positions:

two positions when plaques centered on the equator (tem-

poral and nasal), and two positions in the midway between

the equator and posterior pole (inferior and superior)

(Fig. 2). To investigate the effect of plaque backing and

Acrylic insert on dose rate at points of interest, other Monte

Carlo simulations were employed by replacing the plaque

backing and Acrylic insert by water. Table 4 tabulates the

dose (in Gy) at points of interest for different plaque

materials of 15 mm ROPES plaque fully loaded with

model Theragenics200 and IR06-103Pd seeds. To calculate

the dose at interest points, the air kerma strength for each

seed (SK), is that required to deliver 85 Gy to the tumor

apex in 168 h in water medium. Monte Carlo simulations

were done for different position of the eye plaque. The

results are compared with the dose at the same points when

COMS plaque loaded with Theragenics200 and 6711-125I

seeds are used. According to the Table 4, if the plaque

centered on the equator nasal to the eye ball, moved to

equator temporal the dose at the optic disk decrease in

order of 40%. For the plaque position between the posterior

pole and equator temporal to the eyeball, dose decrease due

to the stainless steel backing and Acrylic insert related to

water medium is about 7.4% for IR06-103Pd seed and 21%

for model Theragenics200. Thomson et al. reported 21%

dose decrease for 103Pd seeds at the opposite side of the eye

to the COMS plaque [26]. Dose at prescription point

(Apex) increases by 4.4% when IR06-103Pd seed is used

instead of model Theragenics200. Due to the presence of

Silastic insert doses at interest points from COMS plaque

loaded with model 200 seeds are lower than doses from

ROPES plaque loaded with the same seed in Acrylic insert.

Comparing to the dose of interest points by 125I source,

doses are consistently lower for all 103Pd seeds. The effect

of stainless steel and Acrylic combination on dose at

interest points is close to the effect of stainless steel

backing only with no Acrylic insert.

Conclusion

The dosimetric parameter values of the new palladium

source are in acceptable agreement in compare with other

Table 4 Doses in ‘‘Gy’’ at points of interest for 15 mm ROPES eye

plaque fully loaded with 10 IR06-103Pd and model Theragenics200

(103Pd) seeds in compare with the doses at the same points for model

Theragenics200 (103Pd) and model 6711 (125I), in 16 mm COMS eye

plaque. Eye plaque centered in the mid way of equator and posterior

pole (p.p) and centered on equator temporal and nasal

Points of interest 15 mm ROPES 16 mm COMS

IR06-103Pda Theragenics 200a Theragenics

200b6711-125Ic

Water

medium

Acrylic

insert only

Stainless steel

backing only

Stainless

steel ? Acrylic

Water

medium

Stainless

steel ? Acrylic

Gold ? Silastic

Center of eye 28.84 28.78 25.72 24.86 25.12 20.13 18.3 23.79

Macula (equator &

p.p)

19.54 19.49 13.00 13.31 13.11 9.07 8.09 12.82

Optic disk

(temporal)

8.76 8.57 6.73 6.16 8.11 6.32 5.32 8.98

Optic disk (nasal) 19.29 18.87 15.78 15.16 23.78 15.51 14.1 21.02

Center of lens

(equator & p.p)

16.21 15.71 14.44 13.28 18.11 13.75 12.5 17.58

Sclera 308.81 308.82 311.88 309.42 317.21 232.70 211 222.9

Apex 85.00 80.00 79.15 79.07 85.00 75.57 68.7 74.43

Lacrima

gland(nasal)

4.10 3.83 3.88 4.12 4.61 3.33 3.03 –

Opposite side 3.23 3.20 3.00 2.99 4.12 3.23 2.94 5.55

a Present work, b Reference [26], c Reference [19]


123


commercial source models the differences are less than 3%.

Two 103Pd seed models, IR06-103Pd and model Thera-

genics200, modeled in ROPES eye plaque to calculate the

dose rate along the central axis and also at the points of

interest. The effect of the plaque backing and Acrylic insert

on dose distribution around the eye plaque is investigated.

The Acrylic insert has very small effect on the dose dis-

tribution and the presence of Acrylic insert can be negligible.

The presence of stainless steel plaque backing enhance the

dose near the plaque due to the secondary produced fluo-

rescence photons and significantly decrease at distances

greater than 2 mm. The dose decrease significantly depends

on the plaque backing composition and also the spectrum of

emitted photons from the seeds. The combination of stainless

steel and Acrylic insert decrease the dose relative to water of

12% at 1 cm on the plaque central axis. Doses to interest

points including the macula, optic disk, lens, sclera and

lacrimal gland has been determined; and also the effect of

plaque backing and Acrylic insert on dose rate at these crit-

ical organs, is investigated. Calculated dose rate at points of

interest and along the central axis of ROPES eye plaque

loaded with 103Pd sources, is higher than the same data for

COMS eye plaque loaded with 103Pd seeds.

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Documents

ROPES eye plaque brachytherapy dosimetry for two models of 103Pd seeds