Monte Carlo Models for Eye Plaque Brachytherapy Treatment of Iris Melanoma

Eye plaque brachytherapy is an effective treatment for eye cancer that uses radioactive seeds (that emit low-energy photons) in an applicator. The purpose of this work is to model eye plaques used to treat iris melanoma and evaluate radiation dose (energy deposited per unit mass) using Monte Carlo simulations. The simulations are performed using the egs_brachy code using 125I (OncoSeed 6711) and 103Pd (TheraSeed 200) brachytherapy seeds. The plaques span 180°, 270°and 360° arcs and are modelled at the center of a 30 cm3 water volume. Dose was scored in a 2.55 cm3 sub-volume made up of 0.5mm3 voxels for simulations with 1010 histories. Voxelized three-dimensional (3D) dose distributions for each plaque are calculated and compared to published BrachyDose data (Thomson et al., 2010). egs_brachy calculated doses are compared to BrachyDose considering absolute dose in gray, dose as a percentage of prescription dose, and dose relative to the TG43 approach currently used in hospitals (Rivard et al., 2004). Statistical analyses show agreement within a 95% confidence interval with published data at 6 points of interest in the eye (cornea, sclera, lens, eye center, macula, and optic disk). Along the plaque central axis, egs_brachy and BrachyDose results agree within statistical uncertainties. In conclusion, eye plaques used for treatment of iris melanoma were successfully modelled and benchmarked. These models will be distributed (open source) with the egs_brachy code on GitHub, enabling state-of-the-art dose calculations in hospitals that will improve patient treatments.