Brachytherapy treatment simulation of strontium-90 and ruthenium-106 plaques on small size posterior uveal melanoma using MCNPX code

Concave eye applicators with ⁹⁰Sr/⁹⁰Y and ¹⁰⁶Ru/¹⁰⁶Rh beta-ray sources are usually used in brachytherapy for the treatment of superficial intraocular tumors as uveal melanoma with thickness up to 5 mm. The aim of this work consisted in using the Monte Carlo code MCNPX to calculate the 3D dose distribution on a mathematical model of the human eye, considering ⁹⁰Sr/⁹⁰Y and ¹⁶⁰Ru/¹⁶⁰Rh beta-ray eye applicators, in order to treat a posterior uveal melanoma with a thickness 3.8 mm from the choroid surface. Mathematical models were developed for the two ophthalmic applicators, CGD produced by BEBIG Company and SIA.6 produced by the Amersham Company, with activities 1 mCi and 4.23 mCi respectively. They have a concave form. These applicators' mathematical models were attached to the eye model and the dose distributions were calculated using the MCNPX *F8 tally. The average doses rates were determined in all regions of the eye model. The *F8 tally results showed that the deposited energy due to the applicator with the radionuclide ¹⁰⁶Ru/¹⁰⁶Rh is higher in all eye regions, including tumor. However the average dose rate in the tumor region is higher for the applicator with ⁹⁰Sr/⁹⁰Y, due to its high activity. Due to the dosimetric characteristics of these applicators, the PDD value for 3 mm water is 73% for the ¹⁰⁶Ru/¹⁰⁶Rh applicator and 60% for ⁹⁰Sr/⁹⁰Y applicator. For a better choice of the applicator type and radionuclide it is important to know the thickness of the tumor and its location.