Baumann Kilian-Simon, Kaupa Sina, Bach Constantin, Engenhart-Cabillic Rita, Zink Klemens
University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany; University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany; Marburg Ion-Beam Therapy Center (MIT), Marburg, Germany.
University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany.
Z Med Phys. 2021 May;31(2):175-191. doi: 10.1016/j.zemedi.2020.08.004. Epub 2021 Mar 26.
Current dosimetry protocols for clinical protons using air-filled ionization chambers assume that the perturbation correction factor is equal to unity for all ionization chambers and proton energies. Since previous Monte Carlo based studies suggest that perturbation correction factors might be significantly different from unity this study aims to determine perturbation correction factors for six plane-parallel and four cylindrical ionization chambers in proton beams at clinical energies.
The dose deposited in the air cavity of the ionization chambers was calculated with the help of the Monte Carlo code TOPAS/Geant4 while specific constructive details of the chambers were removed step by step. By comparing these dose values the individual perturbation correction factors p, p, p, p, p⋅p as well as the total perturbation correction factor p were derived for typical clinical proton energies between 80 and 250MeV.
The total perturbation correction factor p was smaller than unity for almost every ionization chamber and proton energy and in some cases significantly different from unity (deviation larger than 1%). The maximum deviation from unity was 2.0% for cylindrical and 1.5% for plane-parallel ionization chambers. Especially the factor p was found to differ significantly from unity. It was shown that this is due to the fact that secondary particles, especially alpha particles and fragments, are scattered from the chamber wall into the air cavity resulting in an overresponse of the chamber.
Perturbation correction factors for ionization chambers in proton beams were calculated using Monte Carlo simulations. In contrast to the assumption of current dosimetry protocols the total perturbation correction factor p can be significantly different from unity. Hence, beam quality correction factors [Formula: see text] that are calculated with the help of perturbation correction factors that are assumed to be unity come with a corresponding additional uncertainty.
当前使用充气电离室的临床质子剂量测定协议假定，对于所有电离室和质子能量，扰动校正因子都等于1。由于先前基于蒙特卡罗的研究表明，扰动校正因子可能与1有显著差异，因此本研究旨在确定临床能量下质子束中六个平行板电离室和四个圆柱形电离室的扰动校正因子。
借助蒙特卡罗代码TOPAS/Geant4计算电离室气腔内沉积的剂量，同时逐步去除电离室的特定结构细节。通过比较这些剂量值，得出了典型临床质子能量在80至250MeV之间的各个扰动校正因子p、p、p、p、p·p以及总扰动校正因子p。
几乎每个电离室和质子能量下，总扰动校正因子p都小于1，在某些情况下与1有显著差异（偏差大于1%）。圆柱形电离室与1的最大偏差为2.0%，平行板电离室为1.5%。尤其发现因子p与1有显著差异。结果表明，这是由于次级粒子，特别是α粒子和碎片，从室壁散射到气腔中，导致电离室过度响应。
使用蒙特卡罗模拟计算了质子束中电离室的扰动校正因子。与当前剂量测定协议的假设相反，总扰动校正因子p可能与1有显著差异。因此，借助假定为1的扰动校正因子计算的束流质量校正因子[公式：见原文]会带来相应的额外不确定性。
