ENEA, National Institute of Ionizing Radiation Metrology, Rome, Italy.
Phys Med Biol. 2020 May 28;65(9):09NT01. doi: 10.1088/1361-6560/ab79c2.
The high dose and dose-per-pulse rates (up to 130 mGy/pulse) produced by some intraoperative radiation therapy (IORT) accelerators pose specific dosimetric problems due to the high density of electric charge per pulse produced in the ionization chamber cavity. In particular, the correction factor for ion recombination, k , calculated with the traditional two-voltage method is significantly overestimated and three alternative models have been proposed in the literature allowing for the presence of a free-electron component. However, at present there is no general consensus on the best model to assess the ion recombination correction and controversy remains on the uncertainty associated with k . In the present work we adopted a Monte Carlo (MC) approach to assess the uncertainty associated with the ion recombination correction in plane-parallel chambers used in high dose-per-pulse electron beam dosimetry. The uncertainty associated with k was calculated for the following plane-parallel ionization chambers: Scanditronix/Wellhofer Parallel Plate Chamber PPC05 and PPC40, PTW Advanced Markus Model 34 045 and PTW Roos Model 34 001. Input variables for MC calculations were derived from experimental data at 28 and 73 mGy/pulse. Taken together, the results of this study indicate that k values calculated according to the three ion recombination models do not overlap within their standard uncertainties, suggesting that an additional type-B uncertainty component would be necessary to take into account possible differences between the models. Our results indicate that the combined relative standard uncertainty in k should be calculated as the sum in quadrature of a (type-A) MC-based uncertainty component and a (type-B) uncertainty contribution evaluated assuming a uniform distribution between k values obtained from the two extreme models.
由于在电离室腔中每脉冲产生的电荷量很高，一些术中放射治疗（IORT）加速器产生的高剂量和剂量脉冲率（高达 130 mGy/脉冲）带来了特定的剂量学问题。特别是，用传统的双电压法计算的离子复合修正因子 k 被大大高估，文献中提出了三种替代模型，允许存在自由电子分量。然而，目前对于评估离子复合修正的最佳模型尚无共识，并且 k 相关的不确定性仍然存在争议。在本工作中，我们采用蒙特卡罗（MC）方法评估了在高剂量脉冲电子束剂量测定中使用的平面平行室中离子复合修正的不确定性。为以下平面平行电离室计算了 k 相关的不确定性：Scanditronix/Wellhofer 平行板室 PPC05 和 PPC40、PTW 高级 Markus 模型 34045 和 PTW Roos 模型 34001。MC 计算的输入变量来自 28 和 73 mGy/脉冲的实验数据。综上所述，这项研究的结果表明，根据三种离子复合模型计算的 k 值在其标准不确定度内不重叠，表明需要额外的 B 类不确定度分量来考虑模型之间可能存在的差异。我们的结果表明，k 的组合相对标准不确定度应作为基于 MC 的（A 类）不确定性分量的和方和假设在从两个极端模型获得的 k 值之间均匀分布的（B 类）不确定性贡献的和方来计算。
