3.3 Effect of the gold backing

The effect of 20 mm gold plaque backing on dose distribution along the central axis is shown in Figure 7, which provides central axis depth dose curve for full loaded IR06-103Pd, Theragenics 200 and model 2335 seeds with water replacing the Silastic. This figure demonstrates dose is increased near the plaque; now this wellknown effect is due to L-shell fluorescence photons emitted by atoms in the plaque backing [27]. Emission photons from palladium seeds with an average energy of about 21 keV excite the L-shell in gold and silver [31] 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

Central axis depth (mm) a. Gold backing + Silastic

Modelling, Simulation and Dosimetry of 103-Pd Eye Plaque Brachytherapy

DOI: http://dx.doi.org/10.5772/intechopen.88144

Central axis depth (mm) b. Gold backing + Silastic

31

 640.1 479.9 406.9 279.0 306.2 272.0 310.3 0.5 547.0 423.8 368.7 277.7 281.2 252.7 278.1 452.4 366.5 322.4 259.1 250.6 227.6 242.1 0.5 376.3 313.1 278.8 234.2 221.7 204.6 213.0 313.8 268.0 241.9 210.4 198.5 183.8 187.9 1.5 260.6 230.2 211.9 189.1 177.8 166.5 166.2 218.2 199.1 185.4 169.0 160.7 151.3 150.4 2.5 183.8 171.0 162.0 151.3 143.9 137.2 136.7 155.5 147.1 142.5 134.1 129.9 124.9 123.6 113.4 111.0 109.6 107.2 105.5 102.9 102.4 5 (Apex) 85.0 85.0 85.0 85.0 85.0 85.0 85.0 64.7 65.8 67.2 68.1 69.8 70.2 70.8 49.7 51.8 53.4 54.3 56.8 57.8 58.6 39.3 41.3 42.7 44.1 46.6 48.3 48.7 31.2 32.7 34.4 36.1 38.1 40.0 40.9 25.2 26.1 28.1 29.4 31.9 33.1 34.6 SK(U) 11.057 7.299 4.809 5.062 3.405 3.139 3.738

 582.3 462.1 411.1 325.7 309.1 297.4 300.3 0.5 474.1 401.5 375.2 305.5 296.3 232.3 298.5 380.7 325.8 302.1 271.5 264.3 212.8 254.7 0.5 312.5 300.0 267.4 250.7 220.2 202.8 222.8 302.7 248.6 220.2 243.9 189.2 176.4 196.8 1.5 254.8 220.1 215.3 214.4 180.5 167.8 178.7 198.4 190.5 170.0 188.9 168.7 159.8 168.2 2.5 180.8 183.7 162.5 164.3 152.4 140.6 154.6 148.9 150.3 130.8 148.2 132.8 122.7 155.2 120.3 105.1 110.2 120.4 108.7 100.2 104.5 5 (Apex) 85.0 85.0 85.0 85.0 85.0 85.0 85.0 63.7 64.1 72.5 80.0 78.2 69.5 72.4 50.0 50.0 52.4 69.2 60.5 58.4 60.1 32.4 35.2 35.3 55.4 48.9 55.1 51.2 28.4 29.8 34.8 40.5 41.5 38 40.5 22.5 24.6 28.9 28.8 33.1 35.5 34.4 SK(U) 10.88 7.11 5.1 5.23 3.52 3.33 3.61

IR06-103Pd seeds COMS eye plaque diameter (mm) 10 12 14 16 18 20 22

103Pd model 200 seeds [14] COMS eye plaque diameter (mm) 10 12 14 16 18 20 22

#### Figure 5.

Comparison of the calculated anisotropy function of the IR06-103Pd seed versus other available sources at 2 cm [18, 30].

0.7% dose enhancement is observed near the plaque which is loaded with IR06-103Pd seeds, but as the fluorescence photons are absorbed (mean free path is about 2 mm), after a few millimeters, the dose decreases in the order of 6.5%. In Thomson et al. [17] work, a dose decrease of about 6–6.3% at the opposite side of the eye for 103-Pd (Theragenics model 200) seed in gold backing (no Silastic) was reported. Chiu-Tsao et al. reported a dose decrease of about 10% for 125I (model 6711) seed with 20 mm gold plaque (no Silastic) at 7.6 mm. Since the emitted photons from the 125I seed have higher energy than those emitted by 103-Pd seed, more fluorescence photons are 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 the seed and its active length and also depends on the composition of plaque backing.

#### 3.4 Effect of silastic insert

The central axis doses for the IR06-103Pd seeds in Silastic insert with plaque backing are shown in Figure 8 relative to the doses for the same seeds in the water medium. Silastic with an effective atomic number of 10.7 has a greater attenuating effect than water with an effective atomic number of Zeff (7.4) [17]. The average variation in dose distribution due to Silastic insert relative to water is about 17%. Thomson et al. [27] reported 17% dose reduction for Theragenics model 200103-Pd seed at a distance of 1 cm in COMS plaque due to the presence of Silastic insert. Chiu-Tsao et al. [16] calculated a 10% dose reduction at 1 cm for Silastic insert only, in 20 mm COMS eye plaque for 125I, and 16% for 103-Pd (without gold backing) relative to water along the central axis; according to their study also, they presented that the effect of gold+ Silastic combination is comparable with the effect of Silastic insert only. The reduction of dose for 125I source in Silastic insert is more than 103-Pd source due to its higher energy of emitted photons. The dose reduction for the gold + Silastic combination relative to water medium as shown in Figures 6 and 8 is


## Modelling, Simulation and Dosimetry of 103-Pd Eye Plaque Brachytherapy DOI: http://dx.doi.org/10.5772/intechopen.88144

0.7% dose enhancement is observed near the plaque which is loaded with

Comparison of the calculated anisotropy function of the IR06-103Pd seed versus other available sources

Theory, Application, and Implementation of Monte Carlo Method in Science and Technology

plaque backing.

30

Figure 5.

at 2 cm [18, 30].

3.4 Effect of silastic insert

IR06-103Pd seeds, but as the fluorescence photons are absorbed (mean free path is about 2 mm), after a few millimeters, the dose decreases in the order of 6.5%. In Thomson et al. [17] work, a dose decrease of about 6–6.3% at the opposite side of the eye for 103-Pd (Theragenics model 200) seed in gold backing (no Silastic) was reported. Chiu-Tsao et al. reported a dose decrease of about 10% for 125I (model 6711) seed with 20 mm gold plaque (no Silastic) at 7.6 mm. Since the emitted photons from the 125I seed have higher energy than those emitted by 103-Pd seed, more fluorescence photons are 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 the seed and its active length and also depends on the composition of

The central axis doses for the IR06-103Pd seeds in Silastic insert with plaque backing are shown in Figure 8 relative to the doses for the same seeds in the water medium. Silastic with an effective atomic number of 10.7 has a greater attenuating effect than water with an effective atomic number of Zeff (7.4) [17]. The average variation in dose distribution due to Silastic insert relative to water is about 17%. Thomson et al. [27] reported 17% dose reduction for Theragenics model 200103-Pd seed at a distance of 1 cm in COMS plaque due to the presence of Silastic insert. Chiu-Tsao et al. [16] calculated a 10% dose reduction at 1 cm for Silastic insert only, in 20 mm COMS eye plaque for 125I, and 16% for 103-Pd (without gold backing) relative to water along the central axis; according to their study also, they presented that the effect of gold+ Silastic combination is comparable with the effect of Silastic insert only. The reduction of dose for 125I source in Silastic insert is more than 103-Pd source due to its higher energy of emitted photons. The dose reduction for the gold + Silastic combination relative to water medium as shown in Figures 6 and 8 is


#### Table 4.

Central axis dose distributions: (a) in 10–22 mm diameter COMS eye plaques (gold backing + Silastic insert) loaded with IR06-103Pd seeds, (b) gold backing + Silastic insert loaded with 103Pd model 200 seeds [14] and (c) the ratio of the central axis dose of the 10–22 mm COMS plaque (gold backing + silastic) to the central axis dose in water medium loaded with IR06-103Pd seeds.

about 19% at 1 cm and 17% and at 0.5 cm, and the average dose reduction due to the presence of gold backing + Silastic insert along the COMS central axis loaded with the new palladium seeds is about 18%. Thomson et al. have obtained a reduction of the dose relative to water of 20% for 103-Pd seed at 1 cm in the COMS plaque central axis; the main reduction is due to the Silastic insert. The comparison shows that the dose reduction with IR06-103Pd seeds is lower along the central axis of the plaque with the exception of sclera than the other two palladium seed models, Theragenics 200 and model 2335 loaded in COMS plaque.

medium, the air kerma strength for each seed, (SK), is required to deliver 85 Gy to the tumour apex in 168 hours. All of the results have been renormalized to deliver the same dose (85 Gy) to the apex of the tumour. The results are compared with the dose at the same points when COMS plaque is loaded with Theragenics model 200 and 6711-125I seeds. According to Table 5, dose decreases at the optic disk by 40% when moving a plaque from nasal to temporal equatorial centers. This is due to the fact that the optic disk is not centered on the eye anterior-posterior (AP) axis and is nearer the nasal plaque. For the plaque position between the posterior pole and equator temporal to the eyeball, the decrease in dose is due to the gold backing, and Silastic insert related to water medium is about 14% for IR06-103Pd seed and 21% for the model 200 at the opposite side of the eye [17]. When compared to identical plaques loaded with model 6711 125I sources, the doses at points of interest are consistently lower in plaque loaded with any of the 103-Pd models in this study.

Ratio of the doses along the plaque's central axis for 20 mm COMS plaque (no Silastic insert) loaded with

Ratio of the doses along the plaque's central axis for 20 mm COMS plaque fully loaded with IR06-103Pd to the

Modelling, Simulation and Dosimetry of 103-Pd Eye Plaque Brachytherapy

DOI: http://dx.doi.org/10.5772/intechopen.88144

IR06-103Pd, model 200 and model 2335 seeds to the doses in water medium.

Figure 7.

33

Figure 6.

doses in water medium.

## 3.5 Dose comparison at points of interest

To determine the effect of plaque backing and Silastic insert on dose rate at points of interest, more Monte Carlo simulations were employed by replacing the plaque backing and Silastic inserts with water. Table 5 presents the dose (in Gy) at points of interest for different plaque materials of 20 and 16 mm COMS plaque fully loaded with IR06-103Pd seeds. To obtain the dose at the points of interest in a water Modelling, Simulation and Dosimetry of 103-Pd Eye Plaque Brachytherapy DOI: http://dx.doi.org/10.5772/intechopen.88144

#### Figure 6.

Ratio of the doses along the plaque's central axis for 20 mm COMS plaque fully loaded with IR06-103Pd to the doses in water medium.

Figure 7.

about 19% at 1 cm and 17% and at 0.5 cm, and the average dose reduction due to the presence of gold backing + Silastic insert along the COMS central axis loaded with the new palladium seeds is about 18%. Thomson et al. have obtained a reduction of the dose relative to water of 20% for 103-Pd seed at 1 cm in the COMS plaque central axis; the main reduction is due to the Silastic insert. The comparison shows that the dose reduction with IR06-103Pd seeds is lower along the central axis of the plaque with the exception of sclera than the other two palladium seed models, Theragenics

Central axis dose distributions: (a) in 10–22 mm diameter COMS eye plaques (gold backing + Silastic insert) loaded with IR06-103Pd seeds, (b) gold backing + Silastic insert loaded with 103Pd model 200 seeds [14] and (c) the ratio of the central axis dose of the 10–22 mm COMS plaque (gold backing + silastic) to the central

Central axis depth (mm) c. Gold backing + Silastic/water medium

Theory, Application, and Implementation of Monte Carlo Method in Science and Technology

 0.74 0.72 0.71 0.65 0.68 0.66 0.71 0.5 0.75 0.75 0.73 0.72 0.73 0.72 0.73 0.76 0.76 0.76 0.73 0.76 0.73 0.76 0.5 0.78 0.78 0.78 0.77 0.77 0.76 0.77 0.80 0.80 0.79 0.79 0.79 0.79 0.78 1.5 0.81 0.81 0.81 0.81 0.8 0.79 0.78 0.81 0.81 0.81 0.81 0.81 0.80 0.78 2.5 0.81 0.81 0.81 0.81 0.81 0.80 0.79 0.81 0.81 0.81 0.81 0.82 0.80 0.81 0.81 0.81 0.81 0.81 0.82 0.80 0.81 5 (Apex) 0.81 0.81 0.81 0.81 0.82 0.81 0.81 0.80 0.80 0.81 0.8 0.82 0.81 0.81 0.80 0.80 0.81 0.8 0.82 0.81 0.81 0.80 0.80 0.81 0.8 0.82 0.81 0.81 0.80 0.80 0.81 0.81 0.81 0.83 0.80 0.80 0.80 0.81 0.81 0.81 0.83 0.80 SK(U) 8.87 6.12 4 4.13 2.75 2.45 3.07

IR06-103Pd seeds COMS eye plaque diameter (mm) 10 12 14 16 18 20 22

To determine the effect of plaque backing and Silastic insert on dose rate at points of interest, more Monte Carlo simulations were employed by replacing the plaque backing and Silastic inserts with water. Table 5 presents the dose (in Gy) at points of interest for different plaque materials of 20 and 16 mm COMS plaque fully loaded with IR06-103Pd seeds. To obtain the dose at the points of interest in a water

200 and model 2335 loaded in COMS plaque.

axis dose in water medium loaded with IR06-103Pd seeds.

Table 4.

32

3.5 Dose comparison at points of interest

Ratio of the doses along the plaque's central axis for 20 mm COMS plaque (no Silastic insert) loaded with IR06-103Pd, model 200 and model 2335 seeds to the doses in water medium.

medium, the air kerma strength for each seed, (SK), is required to deliver 85 Gy to the tumour apex in 168 hours. All of the results have been renormalized to deliver the same dose (85 Gy) to the apex of the tumour. The results are compared with the dose at the same points when COMS plaque is loaded with Theragenics model 200 and 6711-125I seeds. According to Table 5, dose decreases at the optic disk by 40% when moving a plaque from nasal to temporal equatorial centers. This is due to the fact that the optic disk is not centered on the eye anterior-posterior (AP) axis and is nearer the nasal plaque. For the plaque position between the posterior pole and equator temporal to the eyeball, the decrease in dose is due to the gold backing, and Silastic insert related to water medium is about 14% for IR06-103Pd seed and 21% for the model 200 at the opposite side of the eye [17]. When compared to identical plaques loaded with model 6711 125I sources, the doses at points of interest are consistently lower in plaque loaded with any of the 103-Pd models in this study.

4. Conclusion

geometries.

125-I seeds.

Author details

Sciences, Tehran, Iran

Pooneh Saidi<sup>1</sup>

35

In this study, the dosimetry of a new design brachytherapy seed IR06-103Pd was determined by Monte Carlo using MCNP (version 5). Simulations were considered in three seed orientations with the result that there are no significant statistical differences among the orientations (i.e., ideal, vertical and diagonal). The dosimetric parameters of the new seed are presented in TG-43U1 format. These parameters are in acceptable agreement with those of other commercially available seed models. Thus, the IR06-103Pd seed is dosimetrically suitable for use in routine brachytherapy where the other similar seeds are employed. Also in this study, the dosimetry of IR06-103Pd seed was evaluated in COMS eye plaques and compared to

results for commercially available 103-Pd and 125I seeds in the same plaque

Modelling, Simulation and Dosimetry of 103-Pd Eye Plaque Brachytherapy

DOI: http://dx.doi.org/10.5772/intechopen.88144

seeds and the fluorescence of the backing material.

\* and Mahdi Sadeghi<sup>2</sup>

\*Address all correspondence to: poonehsaidi@gmail.com

provided the original work is properly cited.

1 Parsikan Iran Engineering and Management Consultants, Tehran, Iran

2 Medical Physics Department, School of Medicine, Iran University of Medical

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

relative to water by 19% at 1 cm along the plaque central axis.

The COMS Silastic insert has a significant effect in reducing dose along the plaque central axis. The presence of gold backing enhances the dose near the plaque gold surfaces. This effect is due to secondary fluorescence photons arising from the backing material. Due to the energy of these emissions, the effect is significantly attenuated at a distance >2 mm. The dose decrease depends on the composition of the plaque backing material and therefore on the emitted photon spectrum of the

The combination of gold backing with the Silastic insert decreases the dose

This study shows that this 19% effect is lower than for either model 200 or 2335 103-Pd seeds. Doses to interest points including the macula, optic disk, lens, sclera and lacrimal gland have been determined; and also the effects of plaque backing material and Silastic insert have also been studied at these points. The study affirms that dose and dose rate at these points of interest in COMS plaques are routinely lower when using 103-Pd rather than 125-I seeds. The dose to the proximal sclera, underlying the plaque, is greater using 103-Pd seeds due to lessened penetration than

#### Figure 8.

Ratio of the doses along the plaque's central axis for 20 mm COMS plaque (gold backing + Silastic insert) fully loaded with IR06-103Pd, model 200 and model 2335 seeds [17] to the doses in water medium.



c Ref. [27].

#### Table 5.

Doses in grey at points of interest for (a) 20 mm (b) 16 mm COMS eye plaque loaded with IR06-103Pd seeds compared with the doses at the same points for model 200 (103Pd) and model 6711 (125I), in 16 mm COMS eye plaque. Eye plaque centerd in the midway of equator and posterior pole (equator and p.p) and centerd on equator temporal and nasal.

Modelling, Simulation and Dosimetry of 103-Pd Eye Plaque Brachytherapy DOI: http://dx.doi.org/10.5772/intechopen.88144
