Mills M D, Hogstrom K R, Almond P R
Med Phys. 1982 Jan-Feb;9(1):60-8. doi: 10.1118/1.595138.
A method to predict square and rectangular field output factors from the measurement of selected fields of electron beams on the Therac 20 Saturne has been developed. A two parameter fit of the square field output factor data, based on the functional dependence as predicted by a pencil beam calculational model, has proven clinically acceptable. The pencil beam distributions are given by the Fermi-Eyges theory of multiple Coulomb scattering. For a rectangular field, the output factor can be calculated from the square root of the product of the two square field output factors wtih sides equal to those of the rectangular field. If however, there is a significant asymmetry between the X and Y collimator systems, then rectangular field output factors should be predicted from the product of the X and Y one-dimensional output factors. One-dimensional output factors are defined as output factors of rectangular fields where one side remains constant and equal to the side of the square reference field. Measured data indicate either of the two methods of determining rectangular field output factors to be clinically acceptable for the Therac 20, the use of one-dimensional output factors demonstrating greater accuracy. Data show agreement to within approximately 1.5% at electron energies of 6, 9, 13, and 17 MeV.
已开发出一种通过测量Therac 20 Saturne上电子束选定野来预测方形和矩形野输出因子的方法。基于笔形束计算模型预测的函数依赖性对方形野输出因子数据进行双参数拟合，已证明在临床上是可接受的。笔形束分布由多次库仑散射的费米-艾格斯理论给出。对于矩形野，输出因子可根据两个方形野输出因子乘积的平方根来计算，这两个方形野的边长与矩形野的边长相等。然而，如果X和Y准直器系统之间存在显著不对称，则矩形野输出因子应根据X和Y一维输出因子的乘积来预测。一维输出因子定义为矩形野的输出因子，其中一边保持恒定且等于方形参考野的边长。测量数据表明，对于Therac 20，确定矩形野输出因子的两种方法在临床上都是可接受的，使用一维输出因子显示出更高的准确性。数据显示，在6、9、13和17 MeV的电子能量下，误差在约1.5%以内。
