Medical Physics Department, Faculty of Health Sciences, University of the Free State, P.O. Box 339, Bloemfontein, 9300 South Africa.
Med Phys. 2011 May;38(5):2366-73. doi: 10.1118/1.3570579.
Electron radiation therapy is used frequently for the treatment of skin cancers and superficial tumors especially in the absence of kilovoltage treatment units. Head-and-neck treatment sites require accurate dose distribution calculation to minimize dose to critical structures, e.g., the eye, optic chiasm, nerves, and parotid gland. Monte Carlo simulations can be regarded as the dose calculation method of choice because it can simulate electron transport through any tissue and geometry. In order to use this technique, an accurate electron beam model should be used.
In this study, a two point-source electron beam model developed for an Elekta Precise linear accelerator was validated. Monte Carlo data were benchmarked against measured water tank data for a set of regular and circular fields and at 95, 100, and 110 cm source-to-skin-distance. EDR2 Film dose distribution data were also obtained for a paranasal sinus treatment case using a Rando phantom and compared with corresponding dose distribution data obtained from Monte Carlo simulations and a CMS XiO treatment planning system. A partially shielded electron field was also evaluated using a solid water phantom and EDR2 film measurements against Monte Carlo simulations using the developed source model.
The major findings were that it could accurately replicate percentage depth dose and beam profile data for water measurements at source-to-skin-distances ranging between 95 and 110 cm over beam energies ranging from 4 to 15 MeV. This represents a stand-off between 0 and 15 cm. Most percentage depth dose and beam profile data (better than 95%) agreed within 2%/2 mm and nearly 100% of the data compared within 3%/3 mm. Calculated penumbra data were within 2 mm for the 20 x 20 cm2 field compared to water tank data at 95 cm source-to-skin-distance over the above energy range. Film data for the Rando phantom case showed gamma index map data that is similar in comparison with the treatment planning system and the Monte Carlo source model. The gamma index showed good agreement (2%/2 mm) between the Monte Carlo source model and the film data.
Percentage depth dose and beam profile data were in most cases within a tolerance of 2%/2 mm. The biggest discrepancies were in most cases recorded in the first 6 mm of the water phantom. Circular fields showed local dose agreement within 3%/3mm. Good agreement was found between calculated dose distributions for a paranasal sinus case between Monte Carlo, film measurements and a CMS XiO treatment planning system. The electron beam model can be easily implemented in the BEAMnrc or DOSXYZnrc Monte Carlo codes enabling quick calculation of electron dose distributions in complex geometries.
电子放射治疗常用于治疗皮肤癌和浅表肿瘤，特别是在没有千伏治疗设备的情况下。头颈部治疗部位需要精确的剂量分布计算，以最大限度地减少对关键结构的剂量，例如眼睛、视交叉、神经和腮腺。蒙特卡罗模拟可以被视为首选的剂量计算方法，因为它可以模拟电子在任何组织和几何形状中的传输。为了使用该技术，应使用精确的电子束模型。
在这项研究中，验证了一种为 Elekta Precise 线性加速器开发的两点源电子束模型。将蒙特卡罗数据与一组规则和圆形场以及在 95、100 和 110cm 源皮距处的水箱测量数据进行基准测试。还使用 Rando 体模获得了鼻窦治疗病例的 EDR2 胶片剂量分布数据，并与蒙特卡罗模拟和 CMS XiO 治疗计划系统获得的相应剂量分布数据进行了比较。还使用开发的源模型在固体水模体和 EDR2 胶片测量中评估了部分屏蔽电子场，并与蒙特卡罗模拟进行了比较。
主要发现是，它可以准确地复制水测量的百分深度剂量和束流分布数据，范围在 95 到 110cm 之间，能量范围在 4 到 15MeV 之间。这代表了 0 到 15cm 的距离。大多数百分深度剂量和束流分布数据（优于 95%）在 2%/2mm 内一致，近 100%的数据在 3%/3mm 内一致。在上述能量范围内，与水箱数据相比，20x20cm2 场的计算半影数据在 95cm 源皮距处小于 2mm。与治疗计划系统和蒙特卡罗源模型相比，Rando 体模病例的胶片数据显示出类似的伽马指数图数据。伽马指数在蒙特卡罗源模型和胶片数据之间显示出良好的一致性（2%/2mm）。
百分深度剂量和束流分布数据在大多数情况下都在 2%/2mm 的容差范围内。最大的差异记录在水模体的前 6mm 中。圆形场的局部剂量一致性在 3%/3mm 以内。在鼻窦病例的计算剂量分布之间发现了很好的一致性，包括蒙特卡罗、胶片测量和 CMS XiO 治疗计划系统。电子束模型可以很容易地在 BEAMnrc 或 DOSXYZnrc 蒙特卡罗代码中实现，从而可以在复杂的几何形状中快速计算电子剂量分布。
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