Flock S T, Shragge P C
Med Phys. 1987 Mar-Apr;14(2):202-9. doi: 10.1118/1.596071.
The purpose of this study was to design an improved flattening filter for a Therac 20 medical linear accelerator. Profiles of the 18-MV x-ray beam produced by this accelerator measured along the diagonal of a 40 X 40 cm field at a depth of 5 cm were measured, and it was found that there were regions near the corners of the field where the dose was 109% of the central axis dose. An iterative algorithm for designing flattening filters was developed which required, as input, precise measurements of the following data: the unflattened primary beam profile, the fraction of the beam due to contamination radiation arising from interactions of primary photons with the flattening filter and the collimator assemblies, and the attenuation of the primary photons in water and lead as a function of angle from the central axis of the beam. A new flattening filter was designed and profiles of the beam were measured at a number of depths. These measurements showed that the beam was flattened to within +/- 1% out to 24 cm along the diagonal of a 40 X 40 cm field at a depth of 5 cm.
本研究的目的是为一台Therac 20医用直线加速器设计一种改进的均整滤过器。对该加速器产生的18兆伏X射线束在5厘米深度处沿40×40厘米射野对角线的剂量分布进行了测量，结果发现，在射野拐角附近存在剂量为中心轴剂量109%的区域。开发了一种用于设计均整滤过器的迭代算法，该算法需要以下精确测量数据作为输入：未均整的原射线束剂量分布、由原光子与均整滤过器及准直器组件相互作用产生的污染辐射在射束中所占的比例，以及原光子在水中和铅中的衰减随与射束中心轴夹角的变化关系。设计了一种新的均整滤过器，并在多个深度测量了射束的剂量分布。这些测量结果表明，在5厘米深度处，沿40×40厘米射野对角线在24厘米范围内，射束的均整度达到±1%以内。
