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

Background

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.

Purpose

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.

Methods

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.

Results

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.

Conclusion

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