Department of Radiation Oncology, University of Oklahoma Health Sciences Center, 800 N.E. 10th Street, OKCC L100, Oklahoma City, OK 73104, United States.
Department of Radiation Oncology, Lancaster General Hospital, 2102 Harrisburg Pike, Lancaster, PA 17601, United States.
J Med Imaging Radiat Sci. 2022 Jun;53(2):273-282. doi: 10.1016/j.jmir.2022.02.007. Epub 2022 Mar 15.
Although many studies have investigated small electron fields, there are several dosimetric issues that are not well understood. This includes lack of charged particle equilibrium, lateral scatter, source occlusion and volume averaging of the detectors used in the measurement of the commissioning data. High energy electron beams are also associated with bremsstrahlung production that contributes to dose deposition, which is not well investigated, particularly for small electron fields. The goal of this work has been to investigate dosimetric uncertainties associated with small electron fields using dose measurements with different detectors as well as calculations with eMC dose calculation algorithm.
Different dosimetric parameters including output factors, depth dose curves and dose profiles from small electron field cutouts were investigated quantitatively. These dosimetric parameters were measured using different detectors that included small ion chambers and diodes for small electron cutouts with diameters ranging from 15-50mm mounted on a 6 × 6cm cone with beam energies from 6-20MeV.
Large deviations existed between the output factors calculated with the eMC algorithm and measured with small detectors for small electron fields up to 55% for 6MeV. The discrepancy between the calculated and measured doses increased 10%-55% with decreasing electron beam energy from 20 MeV to 6 MeV for 15mm circular field. For electron fields with cutouts 20mm and larger, the measured and calculated doses agreed within 5% for all electron energies from 6-20MeV. For small electron fields, the maximal depth dose shifted upstream and becomes more superficial as the electron beam energy increases from 6-20MeV as measured with small detectors.
Large dose discrepancies were found between the measured and calculated doses for small electron fields where the eMC underestimated output factors by 55% for small circular electron fields with a diameter of 15 mm, particularly for low energy electron beams. The measured entrance doses and d of the depth dose curves did not agree with the corresponding values calculated by eMC. Furthermore, the measured dose profiles showed enhanced dose deposition in the umbra region and outside the small fields, which mostly resulted from dose deposition from the bremsstrahlung produced by high energy electrons that was not accounted for by the eMC algorithm due to inaccurate modeling of the lateral dose deposition from bremsstrahlung.
Electron small field dosimetry require more consideration of variations in beam quality, lack of charged particle equilibrium, lateral scatter loss and dose deposition from bremsstrahlung produced by energetic electron beams in a comprehensive approach similar to photon small field dosimetry. Furthermore, most of the commercially available electron dose calculation algorithms are commissioned with large electron fields; therefore, vendors should provide tools for the modeling of electron dose calculation algorithms for small electron fields.
尽管许多研究都调查了小电子场，但仍有几个剂量学问题尚未得到很好的理解。这包括缺少带电粒子平衡、侧向散射、源遮挡以及用于测量调试数据的探测器的体积平均。高能电子束还与韧致辐射的产生有关，韧致辐射会导致剂量沉积，这一点尚未得到很好的研究，尤其是对于小电子场。这项工作的目的是使用不同的探测器进行剂量测量以及使用 eMC 剂量计算算法进行计算，来研究与小电子场相关的剂量学不确定性。
使用不同的探测器对小电子场的不同剂量学参数进行了定量研究，包括输出因子、深度剂量曲线和小电子场切出的剂量分布。这些剂量学参数使用不同的探测器进行测量，探测器包括小离子室和二极管，用于直径为 15-50mm 的小电子切出，这些切出安装在 6×6cm 的圆锥体上，电子束能量范围为 6-20MeV。
对于小电子场，与使用小探测器测量的输出因子相比，eMC 算法计算的输出因子存在较大偏差，对于 6MeV 电子束最大偏差高达 55%。对于 15mm 圆形场，随着电子束能量从 20MeV 降低到 6MeV，计算剂量与测量剂量之间的差异增加了 10%-55%。对于 20mm 及更大的电子场，所有电子能量（6-20MeV）的测量和计算剂量均在 5%以内。对于小电子场，随着电子束能量从 6-20MeV 增加，小探测器测量的最大深度剂量向源端迁移，变得更浅。
对于小电子场，测量剂量与计算剂量之间存在较大差异，eMC 算法低估了小圆形电子场（直径为 15mm）的输出因子，最大偏差高达 55%，特别是对于低能电子束。测量的入口剂量和深度剂量曲线的 d 值与 eMC 计算的相应值不一致。此外，测量的剂量分布显示在小场的阴影区域和外部区域有增强的剂量沉积，这主要是由于高能电子产生的韧致辐射的剂量沉积所致，由于 eMC 算法对韧致辐射的侧向剂量沉积的建模不准确，因此无法对其进行计算。
与光子小场剂量学类似，电子小场剂量学需要更全面地考虑束质变化、缺少带电粒子平衡、侧向散射损失以及高能电子产生的韧致辐射的剂量沉积。此外，大多数商业可用的电子剂量计算算法都是使用大电子场进行标定的；因此，供应商应提供用于小电子场电子剂量计算算法建模的工具。
