Mackie T R, Scrimger J W
Radiology. 1982 Jul;144(2):403-9. doi: 10.1148/radiology.144.2.6806853.
The 15-MV photon beam of a linear accelerator (Siemens Mevatron 20) was studied for electron and scattered photon contamination. The surface dose, attributable almost entirely to contamination electrons, has a Gaussian lateral distribution, a linear dependence on field width for square fields, and an inverse square dependence on distance from the bottom of the fixed head assembly. This geometrical dependence is consistent with the proposal that the field flattening filter is the main source of electron contamination when accessories are absent. A tissue-maximum-ratio curve in the build-up region for the electron and photon contamination was produced utilizing the linearity of dose with respect to field width. The derived contamination curve inside was similar to the measured build-up curve outside the field. The primary photon component, obtained by subtracting the contaminant contribution, showed no dependence on field size, source-to-probe distance, or presence of accessories.
对一台直线加速器（西门子Mevatron 20）的15兆伏光子束进行了电子和散射光子污染研究。表面剂量几乎完全归因于污染电子，具有高斯横向分布，对于方形野，其与野宽呈线性关系，与距固定机头组件底部的距离呈平方反比关系。这种几何关系与以下观点一致：当没有附件时，场平坦化滤波器是电子污染的主要来源。利用剂量与野宽的线性关系，绘制了电子和光子污染在建成区的组织最大剂量比曲线。推导得到的内部污染曲线与野外测量的建成曲线相似。通过减去污染物贡献得到的初级光子成分，未显示出对野大小、源到探测器距离或附件存在的依赖性。
