Purpose: In radiotherapy, determination and management of delivery dose by measuring the absolute dose are important. The reference dose for clinical proton beams is based on ionization chamber dosimetry. However, there are few data on the beam quality correction factors (k(Q)) required for calculating the absolute dose for the proton beam. Furthermore, the perturbation correction factor (P(Q)) of ionization chambers is assumed to be unity for proton beams in the IAEA TRS-398 CoP. Hence, the aim of this study is to calculate k(Q) and P(Q) for ionization chambers in scanning proton beams using the Monte Carlo simulation (MC).
Methods: We used PTSIM a particle therapy simulation flamework based on MC code Geant4 version 10.05.p01. We reproduced a spot scanning nozzle (PROBEAT-Ⅲ: Hitachi,Ltd) and calculated the f(Q) and P(Q) for the cylindrical ionization chamber (30013: PTW). f is determined by the ratio of the absorbed dose of water D(w) to dose of cavity D(air) in the chamber. We investigated four different monoenergetic beams and one modulated beam (SOBP) with a range of 12 cm. The former was measured at 2 cm, the latter was measured at various depths with different dose gradient.
Results: For the maximum difference in monoenergy, the f(Q) between this work and Baumann et al (2020) was 0.4%. The f(Q) of the modulated proton beam at the SOBP center was 1.1249. At proximal region the f(Q) increased up to about 2.0%. Perturbation correction factor P(cel) was about 0.4% different than unity and was independent in the energy and measurement depth.
Conclusion: We calculated f(Q) and P(cel) for cylindrical ionization chamber in monoenergetic and modulated proton beams. We will investigate other individual components of P(Q) using cylindrical ionization chamber.