Xu M Ming, Sethi Anil, Glasgow Glenn P
Department of Radiation Oncology, Loyola University Medical Center, Maywood, IL 60153, USA.
Med Dosim. 2009 Spring;34(1):51-6. doi: 10.1016/j.meddos.2007.12.003. Epub 2008 Apr 1.
Small field electron beams used in the clinic present complex dosimetry. This investigation reports dosimetric characteristics of small 6-MeV electron field measurements and Monte Carlo calculations, and examines their impact on patient dosimetry. Radiographic film and ionization chamber were used for dosimetric measurements for a 6-MeV electron beam from a Varian 21EX linac. A set of circular cerrobend cutouts with diameters ranging from 2 to 3.0 cm was made. A clinical Monte Carlo algorithm was used to calculate dosimetric parameters. As the cutout diameter decreased, the build-up portion of the percentage depth dose (PDD) curves shifted toward the phantom surface, the depth of maximum dose, d(max,) decreased from 1.2 cm to 0.6 cm, but the practical range, R(p), remained constant at about 3.1 cm. The absorbed dose rate, D(r), decreased linearly with decreasing diameter. Profile coverage ratios CR (ie, the ratio of a given isodose line to the cutout diameter) at d(max), for 90% and 80%, CR(90), CR(80), are approximately 55% and 73%, respectively. As the cutout diameter decreased, the 90% to 10% penumbra to diameter ratio, PR, increased from 0.49 to 0.56 for 3- to 2-cm cutouts. The total 90% to 10% penumbra was about the same size as the cutout diameter. The measured output factors were in good agreement with Monte Carlo calculation within 2.2%. Accurate small electron field measurements were performed using parallel plate ion chamber and film. The data were well supported by Monte Carlo calculations. These data facilitate routine clinical treatments for small cutouts as d(max), D(r), and other data can be quickly obtained instead of performing labor-intensive individual patient measurements.
临床中使用的小射野电子束具有复杂的剂量学特性。本研究报告了小 6 MeV 电子射野测量和蒙特卡罗计算的剂量学特征，并研究了它们对患者剂量学的影响。使用放射照相胶片和电离室对瓦里安 21EX 直线加速器产生的 6 MeV 电子束进行剂量学测量。制作了一组直径范围为 2 至 3.0 cm 的圆形铈基低熔点合金挡块。使用临床蒙特卡罗算法计算剂量学参数。随着挡块直径减小，百分深度剂量（PDD）曲线的剂量建成部分向模体表面移动，最大剂量深度 d(max) 从 1.2 cm 降至 0.6 cm，但实际射程 R(p) 保持在约 3.1 cm 不变。吸收剂量率 D(r) 随直径减小呈线性下降。在 d(max) 处，90% 和 80% 的剂量分布覆盖比 CR（即给定等剂量线与挡块直径之比），CR(90)、CR(80) 分别约为 55% 和 73%。随着挡块直径减小，3 cm 至 2 cm 挡块的 90% 至 10% 半值宽度与直径之比 PR 从 0.49 增加到 0.56。总的 90% 至 10% 半值宽度与挡块直径大小大致相同。测量的输出因子与蒙特卡罗计算结果在 2.2% 范围内吻合良好。使用平行板电离室和胶片进行了精确的小电子射野测量。蒙特卡罗计算很好地支持了这些数据。这些数据有助于小挡块的常规临床治疗，因为可以快速获得 d(max)、D(r) 和其他数据，而无需进行耗费人力的个体患者测量。
