Monte Carlo-calculated perturbation correction factors in clinical proton beams using PHITS

Accurate absolute dosimetry is essential for achieving high-precision proton beam therapy. Consequently, a comprehensive characterization of the ionization chamber's response properties is necessary.

This study aimed to evaluate the average $f_Q$ using Monte Carlo (MC) code PHITS to assess uncertainties among different MC simulation tools. Additionally, $P_Q$ values for PTW 30013, NACP-02, and PTW 31013 ionization chambers are calculated using PHITS to provide new reference data for $P_Q$ . Furthermore, a new $k_Q$ factor for PTW 31013 chamber is established using MC method, contributing to advancements in proton beam dosimetry protocols.

Monoenergetic proton beams were employed to calculate $f_Q$ , $k_Q$ , and $P_Q$ for Farmer, Semiflex, and plane-parallel chambers. The absorbed dose deposited within the sensitive volume of each chamber was determined via simulations employing PHITS, thereby providing the basis for the estimation of these factors. Computed $f_Q$ values were compared with previous reports, while $k_Q$ and $P_Q$ were benchmarked against literature and Technical Reports Series No. 398 (TRS-398) Rev.1 guideline.

Incorporating PHITS-derived $f_Q$ values reduced the uncertainty of ${\overline{f}}_Q^{PHITS}$ compared to previous findings. The $k_Q$ factor for PTW 31013 followed trends observed in cylindrical chambers with varying sensitive volumes; notably, this study represents the first MC estimation of $k_Q$ for this chamber. $P_Q$ values for values deviated by up to 1.7% from unity.

The data generated in this study provide important insights for refining proton beam dosimetry, contributing to the improvement of treatment precision.