Improvement of the hybrid approach between Monte Carlo simulation and analytical function for calculating microdosimetric probability densities in macroscopic matter

Sato, Tatsuhiko   ; Matsuya, Yusuke*  ; Ogawa, Tatsuhiko   ; Kai, Takeshi   ; Hirata, Yuho   ; Tsuda, Shuichi  ; Parisi, A.*

In this study, we improved the microdosimetric function implemented in PHITS using the latest track-structure simulation codes. The improved function is capable of calculating the probability densities of not only the conventional microdosimetric quantities such as lineal energy but also the numbers of ionization events occurred in a target site, the so-called ionization cluster size distribution, for arbitrary site diameters from 3 nm to 1 um. As a new application of the improved function, we calculated the relative biological effectiveness of the single-strand break and double-strand break yields for proton irradiations using the updated PHITS coupled with the simplified DNA damage estimation model, and confirmed its equivalence in accuracy and its superiority in computational time compared to our previously proposed method based on the track-structure simulation.