Microdosimetry study of proton quality factor using analytic model calculations

Papadopoulos, A.*; Kyriakou, I.*; Matsuya, Yusuke  ; Incerti, S.*; Daglis, I. A.*; Emfietzoglou, D.*

The quality factor (Q) is the index which is used when evaluating the stochastic (e.g., carcinogenic) risk of diverse ionizing radiations. While the Q value can be commonly determined from the Linear Energy Transfer (LET), more elaborate approaches are based on the microdosimetric parameter lineal energy (y) calculated either by analytical model or Monte Carlo (MC) simulations. However, the developing of the model for determining the Q value is still ongoing worldwide for realizing the precise risk assessment. In this study, various generalized analytical models that account for both -ray transport and energy-loss straggling effects are utilized to evaluate the Q values over the proton energy range 1-250 MeV. The results revealed that the LET-based ICRP Report 60 recommendations underestimate the microdosimetric-based Q values of protons with energy below 100 MeV, which was also confirmed by using MC simulation data on the y values. The present work shows that analytic models may offer a practical alternative to computer-intensive MC simulations for calculating Q values based on the microdosimetric methodologies. In future study, we are planning to compare the y spectra and subsequent calculations of Q based on new MC data with the latest versions of Geant4-DNA and PHITS track structure codes which make use of different physics models.