Computational Assessment of Radiation Dose Enhancement and Secondary Electron Production for Variable Sizes and Concentrations of Gold Nanospheres in a Tumor using MCNP6.2*

The purpose of this study is to computationally quantify dose enhancement effects of using different concentrations and sizes of gold nanospheres in high dose rate (HDR) brachytherapy and external beam radiotherapy. A MicroSelectron HDR Ir-192 brachytherapy source and a Varian 600C gantry head with a 6MV photon energy were modeled using Monte Carlo N-Particle radiation transport software (MCNP 6.2, Los Alamos National Laboratory). The repeating structures capability of MCNP6.2 was utilized to simulate nanospheres of sizes 4.5 nm, 30 nm and 60 nm at varying nanosphere concentrations of 5 nM, 10 nM and 20 nM, inside a tumor, with a diameter of 1 x 1 x 1 cm³. Dose enhancement factors (DEFs) were computed as the ratio of dose to the tumor containing gold nanospheres relative to that without. The highest DEF of 1.7 was observed with the Ir-192 source for a total nanosphere concentration of 20 nM and diameters of 4.5 nm. It was observed that increasing concentration and decreasing the size of the nanospheres produced the greatest dose enhancement for both HDR brachytherapy and external beam radiotherapy cases. This work indicates the potential for significant dose enhancement and more effective tumor cell killing in radiation oncology practice.