4. Conclusion

Figure 8.

p.p)

p.p)

a Present work. b Ref. [17]. c Ref. [27].

34

Table 5.

Centre of lens (equator and

Centre of lens (equator and

centerd on equator temporal and nasal.

Ratio of the doses along the plaque's central axis for 20 mm COMS plaque (gold backing + Silastic insert) fully

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

Silastic insert only

IR06-103Pda

16.21 18.98 17.23 19.00

Gold backing only

b. 16 mm COMS Water medium Gold backing + Silastic IR06-103Pda Theragenics 200<sup>b</sup> IR06-103Pda Theragenics 200<sup>b</sup> 6711-125I

15.63 16.34 11.83 12.5 17.58

Gold backing + Silastic

c

loaded with IR06-103Pd, model 200 and model 2335 seeds [17] to the doses in water medium.

Centre of eye 29.08 34.31 30.82 34.37 Macula (equator and p.p) 15.57 17.22 16.55 18.29 Optic disk (temporal) 7.52 8.81 7.98 8.85 Optic disk (nasal) 18.29 21.47 19.42 21.44

Sclera 351.81 367.38 374.68 383.73 Apex 85.00 85.00 85.00 85.00 Lacrimal gland (nasal) 4.82 5.59 5.12 5.62 Opposite side 3.39 3.94 3.59 3.95

Centre of eye 22.52 22.75 17.8 18.3 23.79 Macula (equator and p.p) 10.61 11.43 9.8 8.089 12.82 Optic disk (temporal) 6.32 7.193 5.12 5.35 8.98 Optic disk (nasal) 19.63 21.65 13.67 14.13 21.02

Sclera 297.41 287.92 261.4 211 222.9 Apex 85.00 85.00 68.23 68.7 74.43 Lacrimal gland (nasal) 3.72 — 2.75 3.03 — Opposite side 3.63 3.77 2.22 2.94 5.55

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

Points of interest a. 20 mm COMS

Water medium

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 geometries.

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 seeds and the fluorescence of the backing material.

The combination of gold backing with the Silastic insert decreases the dose relative to water by 19% at 1 cm along the plaque central axis.

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 125-I seeds.
