Evaluation of RBE-weighted doses for various radiotherapy beams based on a microdosimetric function implemented in PHITS

University of Tsukuba is developing a new TPS for boron neutron capture therapy (BNCT) equipped with Monte Carlo dose-calculation engine based on Particle and Heavy Ion Transport code System PHITS. It is currently in the process of extending its adaptation to other radiotherapy beams. For this extension, not only physical doses but also their relative biological effectiveness (RBE) must be evaluated for various radiotherapy in the same framework. Frequent and dose probability densities of lineal energy, y, are the key quantities in the RBE estimation, and they must be precisely evaluated for various locations in a patient. In this study, the probability densities of y for a site diameter of 0.564 µm were calculated for X-ray, proton, carbon-ion, and BNCT beams with appropriate geometry settings using the microdosimetric function implemented in PHITS, and they were converted to the corresponding RBE-weighted doses using the microdosimetric kinetic model. The accuracy of the calculated data were well verified by several experimental data, indicating the adequacy of the use of PHITS and microdosimetric kinetic model in the dose-calculation engine for TPS applicable to various radiotherapy.