Paz-Martín José, Schüller Andreas, Bourgouin Alexandra, Gago-Arias Araceli, González-Castaño Diego M, Gómez-Fernández Nicolás, Pardo-Montero Juan, Gómez Faustino
Departamento de Física de Partículas, Universidade de Santiago de Compostela, Santiago de Compostela, A Coruña, Spain.
Dosimetry for Radiotherapy, Physikalisch-Technische Bundesanstalt, Braunschweig, Germany.
Med Phys. 2025 Jun;52(6):4894-4909. doi: 10.1002/mp.17814. Epub 2025 Apr 11.
Air-vented ionization chambers exposed to clinical radiation beams may suffer from recombination during the drift of the charge carriers towards the electrodes. Thus, dosimetry protocols recommend the use of a correction factor, usually denominated saturation factor ( ), to correct the ionization chamber readout for the incomplete collection of charge. The two-voltage method (TVM) is the recommended methodology for the calculation of the saturation factor, however, it is based on the early Boag model, which only takes into account the presence of positive and negative ions in the ionization chamber and does not account for the electric field screening or the free electron contribution to the signal.
To evaluate the impact of a more realistic approach to the saturation problem that accounts for the free electron fraction.
The saturation factor of four ionization chambers (two Advanced Markus and two PPC05) was experimentally determined in the ultra-high dose per pulse reference beam of the German National Metrology Institute (Physikalisch-Technische Bundesanstalt [PTB]) for voltages ranging from 50 to 400 V and pulse durations between 0.5 and 2.9 . Several analytical models and a recently developed numerical model are used to calculate the saturation factor as a function of the dose per pulse and compare it to the obtained experimental data. Parameterizations of the saturation factor against the ratio of charges at different voltages are given for parallel plate ionization chamber with a distance between electrodes of 0.6 and 1 mm in pulsed beams for different pulse durations.
The saturation factors calculated using the different Boag analytical models do not agree neither with each other nor with the numerical simulation even at the lowest dose per pulse of the investigated range ( 30 mGy). A recently developed analytical model by Fenwick and Kumar agrees with the numerical simulation in the low dose per pulse regime but discrepancies are observed when the dose becomes larger (i.e., 40 mGy for Advanced Markus) due to the electric field perturbation. The numerical simulation is in a good agreement with the experimentally determined charge collection efficiency (CCE) with an average discrepancy of 0.7% for the two PPC05 and 0.5% for the two Advanced Markus. The saturation factor obtained with the numerical simulation of the collected charge has been fitted to a third-order polynomial for different voltage ratios and pulse duration. This methodology provides a practical way for evaluation whenever .
The numerical simulation shows a better agreement with the experimental data than the current analytical theories in terms of CCE. The classical TVM, systematically overestimates the saturation factor, with differences increasing with dose per pulse but also present at low dose per pulse. These results may have implications for the dosimetry with ionization chambers in therapy modalities that use a dose per pulse higher than conventional radiotherapy such as intraoperative radiotherapy but also in conventional dose per pulse for ionization chambers that suffer from significant charge recombination.
暴露于临床辐射束的通气电离室在电荷载流子向电极漂移过程中可能会发生复合。因此，剂量测定协议建议使用一个校正因子，通常称为饱和因子（ ），以校正电离室读数因电荷不完全收集而产生的误差。双电压法（TVM）是计算饱和因子的推荐方法，然而，它基于早期的博阿格模型，该模型仅考虑了电离室内正负离子的存在，而没有考虑电场屏蔽或自由电子对信号的贡献。
评估一种更现实的解决饱和问题的方法的影响，该方法考虑了自由电子分数。
在德国国家计量研究所（Physikalisch-Technische Bundesanstalt [PTB]）的超高剂量每脉冲参考束中，对四个电离室（两个高级马克斯电离室和两个PPC05电离室）的饱和因子进行了实验测定，电压范围为50至400 V，脉冲持续时间在0.5至2.9 之间。使用几种分析模型和最近开发的数值模型来计算饱和因子作为每脉冲剂量的函数，并将其与获得的实验数据进行比较。给出了在不同脉冲持续时间的脉冲束中，电极间距为0.6至1 mm的平行板电离室的饱和因子相对于不同电压下电荷比的参数化。
即使在研究范围的最低每脉冲剂量（ 30 mGy）下，使用不同博阿格分析模型计算的饱和因子彼此之间也不一致，与数值模拟结果也不一致。Fenwick和Kumar最近开发的分析模型在低每脉冲剂量范围内与数值模拟结果一致，但当剂量变大时（即高级马克斯电离室为 40 mGy），由于电场扰动会出现差异。数值模拟与实验确定的电荷收集效率（CCE）吻合良好，两个PPC05电离室的平均差异为0.7%，两个高级马克斯电离室的平均差异为0.5%。通过对收集电荷的数值模拟获得的饱和因子已针对不同电压比和脉冲持续时间拟合为三阶多项式。这种方法为 评估提供了一种实用的途径。
在CCE方面，数值模拟比当前的分析理论与实验数据的吻合度更好。经典的TVM系统性地高估了饱和因子，差异随着每脉冲剂量的增加而增大，但在低每脉冲剂量时也存在。这些结果可能对使用高于传统放疗每脉冲剂量的治疗方式（如术中放疗）中的电离室剂量测定有影响，对于遭受显著电荷复合的电离室在传统每脉冲剂量下也有影响。
