Department of Radiation Oncology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, USA.
Department of Engineering and Physics, University of Central Oklahoma, Edmond, Oklahoma, USA.
J Appl Clin Med Phys. 2022 Jan;23(1):e13478. doi: 10.1002/acm2.13478. Epub 2021 Nov 25.
In the electron beam radiation therapy, customized blocks are mostly used to shape treatment fields to generate conformal doses. The goal of this study is to investigate quantitatively dosimetric uncertainties associated with heterogeneities, detectors used in the measurement of the beam data commissioning, and modeling of the interactions of high energy electrons with tissue. These uncertainties were investigated both by measurements with different detectors and calculations using electron Monte Carlo algorithm (eMC) in the Eclipse treatment planning system. Dose distributions for different field sizes were calculated using eMC and measured with a multiple-diode-array detector (MapCheck2) for cone sizes ranging from 6 to 25 cm. The dose distributions were calculated using the CT images of the MapCheck2 and water-equivalent phantoms. In the umbra region (<20% isodose line), the eMC underestimated dose by a factor of 3 for high energy electron beams due to lack of consideration of bremsstrahlung emitted laterally that was not accounted by eMC in the low dose region outside the field. In the penumbra (20%-80% isodose line), the eMC overestimated dose (40%) for high energy 20 MeV electrons compared to the measured dose with small diodes in the high gradient dose region. This was mainly due to lack of consideration of volume averaging of the ion chamber used in beam data commissioning which was input to the eMC dose calculation algorithm. Large uncertainties in the CT numbers (25%) resulted from the image artifacts in the CT images of the MapCheck2 phantom due to metal artifacts. The eMC algorithm used the electron and material densities extracted from the CT numbers which resulted large dosimetric uncertainties (10%) in the material densities and corresponding stopping power ratios. The dose calculations with eMC are associated with large uncertainties particularly in penumbra and umbra regions and around heterogeneities which affect the low dose level that cover nearby normal tissue or critical structures.
在电子束放射治疗中，通常使用定制的挡块来调整射野形状以实现适形剂量。本研究的目的是定量研究与不均匀性、束流数据调试中使用的探测器以及高能电子与组织相互作用建模相关的剂量学不确定性。通过使用不同的探测器进行测量以及在 Eclipse 治疗计划系统中使用电子蒙特卡罗算法（eMC）进行计算，研究了这些不确定性。使用 eMC 计算不同射野大小的剂量分布，并使用多二极管阵列探测器（MapCheck2）进行测量，锥形束大小范围为 6 至 25 cm。使用 MapCheck2 的 CT 图像和水等效体模计算剂量分布。在阴影区域（<20%等剂量线），由于 eMC 未考虑侧向发射的韧致辐射，高能电子束的剂量低估了 3 倍，而该辐射在野外低剂量区域未被 eMC 考虑。在半影区（20%-80%等剂量线），与小二极管在高梯度剂量区域测量的剂量相比，高能 20 MeV 电子的 eMC 高估了 40%的剂量。这主要是由于在将束流数据调试中使用的电离室的体积平均值输入到 eMC 剂量计算算法时未考虑到该因素。MapCheck2 体模的 CT 图像中的金属伪影导致 CT 数的不确定性较大（25%）。eMC 算法使用从 CT 数中提取的电子和材料密度，这导致材料密度和相应的阻止本领比的剂量学不确定性较大（10%）。eMC 计算的剂量特别在半影和阴影区域以及在不均匀性周围存在较大的不确定性，这会影响覆盖附近正常组织或关键结构的低剂量水平。
