Dawson D J
Med Phys. 1977 Sep-Oct;4(5):423-30. doi: 10.1118/1.594328.
The tissue-maximum ratios for high-energy betatron x rays, measured at target-axis distances of 90, 110, and 130 cm, exhibit a strong dependence on the target-axis distance in the buildup region as well as a significant dependence in the exponential region. This dependence and the field-size dependence of the tissue-maximum ratios at a fixed target-axis distance are studied with the use of phantom and collimator scatter factors. The depth and field-size dependence of these scatter factors are determined experimentally. The measurements indicate that the collimator scatter factor is dependent on both the collimator opening and the targetaxis distance. These components are quantitated separately and are associated primarily with high-energy electron scatter from the collimating system. An equation involving these components of the collimator scatter factor is derived for the conversion of tissue-maximum ratios from one target-axis distance to another. With the use of this equation, the tissue-maximum ratios measured for one target-axis distance can be used to calculate within 1% those at other target-axis distances.
在90厘米、110厘米和130厘米的靶轴距离处测量的高能电子感应加速器X射线的组织最大剂量比，在积累区对靶轴距离有强烈依赖性，在指数区也有显著依赖性。利用体模和准直器散射因子研究了这种依赖性以及在固定靶轴距离下组织最大剂量比的射野大小依赖性。通过实验确定了这些散射因子的深度和射野大小依赖性。测量结果表明，准直器散射因子既取决于准直器开口，也取决于靶轴距离。这些分量被分别量化，并且主要与来自准直系统的高能电子散射有关。推导了一个涉及准直器散射因子这些分量的方程，用于将组织最大剂量比从一个靶轴距离转换到另一个靶轴距离。使用这个方程，在一个靶轴距离处测量的组织最大剂量比可用于计算其他靶轴距离处的组织最大剂量比，误差在1%以内。
