Larsen R D, Brown L H, Bjärngard B E
Med Phys. 1978 May-Jun;5(3):215-20. doi: 10.1118/1.594430.
A flattening filter is an important component in a medical accelerator to modify the photonbeam properties. To simplify the calculations of the flattening-filter profile, we have developed a computer program which sums primary and scatter and then makes iterations in the primary component to produce a desired total-dose profile. The program can account, to first order, for radial spectral changes by using an effective primary attenuation coefficient which varies with radius. Calculations made to model the Clinac-4 dose profiles using the measured variation of half-value layer with radius show good agreement with the measured data. It is shown that the variation of quality within the beam impairs the flatness that can be achieved over a range of depths. Since perfect flatness cannot be achieved for small and large fields with one flattening filter, one may choose a primary profile which is a compromise over a range of field sizes and depths. A compromise profile for a 4-MV beam is discussed.
均整滤过器是医用加速器中用于改变光子束特性的重要部件。为简化均整滤过器轮廓的计算，我们开发了一个计算机程序，该程序对原射线和散射线进行求和，然后在原射线分量中进行迭代以生成所需的总剂量轮廓。该程序可以通过使用随半径变化的有效原射线衰减系数，一阶近似地考虑径向光谱变化。利用测量得到的半值层随半径的变化来模拟Clinac - 4剂量轮廓所进行的计算结果与测量数据显示出良好的一致性。结果表明，射束内剂量质的变化会损害在一系列深度上所能达到的平坦度。由于使用一个均整滤过器无法在小射野和大射野上都实现完美的平坦度，因此可以选择一种在一系列射野尺寸和深度范围内折中的原射线轮廓。本文讨论了4兆伏射束的一种折中型轮廓。
