Institute of Medical Physics and Radiation Protection, University of Applied Sciences Giessen, Wiesenstrasse 14, Giessen, D-35390, Germany.
University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany.
Med Phys. 2017 Jun;44(6):2569-2580. doi: 10.1002/mp.12252. Epub 2017 May 20.
The impact of removing the flattening filter in clinical electron accelerators on the relationship between dosimetric quantities such as beam quality specifiers and the mean photon and electron energies of the photon radiation field was investigated by Monte Carlo simulations. The purpose of this work was to determine the uncertainties when using the well-known beam quality specifiers or energy-based beam specifiers as predictors of dosimetric photon field properties when removing the flattening filter.
Monte Carlo simulations applying eight different linear accelerator head models with and without flattening filter were performed in order to generate realistic radiation sources and calculate field properties such as restricted mass collision stopping power ratios (L¯/ρ)airwater, mean photon and secondary electron energies. To study the impact of removing the flattening filter on the beam quality correction factors k , this factor for detailed ionization chamber models was calculated by Monte Carlo simulations. Stopping power ratios (L¯/ρ)airwater and k values for different ionization chambers as a function of TPR1020 and %dd(10) were calculated. Moreover, mean photon energies in air and at the point of measurement in water as well as mean secondary electron energies at the point of measurement were calculated.
The results revealed that removing the flattening filter led to a change within 0.3% in the relationship between %dd(10) and (L¯/ρ)airwater, whereby the relationship between TPR1020 and (L¯/ρ)airwater changed up to 0.8% for high energy photon beams. However, TPR1020 was a good predictor of (L¯/ρ)airwater for both types of linear accelerator with energies < 10 MeV with a maximal deviation between both types of accelerators of 0.23%. According to the results, the mean photon energy below the linear accelerators head as well as at the point of measurement may not be suitable as a predictor of (L¯/ρ)airwater and k to merge the dosimetry of both linear accelerator types. It was possible to derive (L¯/ρ)airwater using the mean secondary electron energy at the point of measurement as a predictor with an accuracy of 0.17%. A bias between k for linear accelerators with and without flattening filter within 1.1% and 1.6% was observed for TPR1020 and %dd(10) respectively.
The results of this study have shown that removing the flattening filter led to a change in the relationship between the well-known beam quality specifiers and dosimetric quantities at the point of measurement, namely (L¯/ρ)airwater, mean photon and electron energy. Furthermore, the results show that a beam profile correction is important for dose measurements with large ionization chambers in flattening filter free beams.
通过蒙特卡罗模拟研究在临床电子加速器中去除滤光器对剂量学量（如束品质指标和光子辐射场的平均光子和电子能量）之间关系的影响。本工作的目的是确定在去除滤光器时使用已知的束品质指标或基于能量的束指标作为剂量学光子场特性的预测因子时的不确定性。
为了生成逼真的辐射源并计算限制质量碰撞阻止本领比（L¯/ρ）airwater、平均光子和次级电子能量等场特性，对带有和不带有滤光器的八种不同线性加速器头模型进行了蒙特卡罗模拟。为了研究去除滤光器对详细电离室模型的束品质校正因子 k 的影响，通过蒙特卡罗模拟计算了这个因子。计算了不同电离室的阻止本领比（L¯/ρ）airwater 和 k 值作为 TPR1020 和 %dd(10)的函数。此外，还计算了空气中的平均光子能量和水中测量点的平均光子能量以及测量点的平均次级电子能量。
结果表明，去除滤光器导致 %dd(10)与（L¯/ρ）airwater 之间的关系发生了 0.3%的变化，而 TPR1020 与（L¯/ρ）airwater 之间的关系对于高能光子束变化了高达 0.8%。然而，对于能量 < 10 MeV 的两种类型的线性加速器，TPR1020 都是（L¯/ρ）airwater 的良好预测因子，两种类型的加速器之间的最大偏差为 0.23%。根据结果，线性加速器头部以下和测量点处的平均光子能量可能不适合作为（L¯/ρ）airwater 和 k 的预测因子，以合并两种线性加速器类型的剂量学。可以使用测量点处的平均次级电子能量作为预测因子，以 0.17%的精度推导出（L¯/ρ）airwater。对于 TPR1020 和 %dd(10)，观察到带有和不带滤光器的线性加速器之间 k 的偏差分别在 1.1%和 1.6%以内。
本研究结果表明，去除滤光器导致已知束品质指标与测量点处剂量学量（即（L¯/ρ）airwater、平均光子和电子能量）之间的关系发生变化。此外，结果表明在去除滤光器的自由束中，剂量测量时进行束流形状校正很重要。
