Ulin K, Sternick E S
Department of Radiation Oncology, Tufts-New England Medical Center, Boston, Massachusetts 02111.
Med Phys. 1989 Nov-Dec;16(6):905-10. doi: 10.1118/1.596316.
It is well known that when an electron beam is incident obliquely on the surface of a phantom, the depth dose curve measured normal to the surface is shifted toward the surface. Based on geometrical arguments alone, the depth of the nth isodose line for an electron beam incident at an angle theta should be equal to the product of cos theta and the depth of the nth isodose line at normal incidence. This method, however, ignores the effects of scatter and can lead to significant errors in isodose placement for beams at large angles of incidence. A semi-empirical functional relationship and a table of isodose shift factors have been developed with which one may easily calculate the depth of any isodose line for beams at incident angles of 0 degrees to 60 degrees. The isodose shift factors are tabulated in terms of beam energy (6-22 MeV) and isodose line (10%-90%) and are shown to be relatively independent of beam size and incident angle for angles less than 60 degrees. Extensive measurements have been made on a Varian Clinac 2500 linear accelerator with a parallel-plate chamber and polystyrene phantom. The dependence of the chamber response on beam angulation has been checked, and the scaling factor of the polystyrene phantom has been determined to be equal to 1.00.
众所周知，当电子束倾斜入射到模体表面时，垂直于表面测量的深度剂量曲线会向表面偏移。仅基于几何原理，以角度θ入射的电子束的第n条等剂量线的深度应等于cosθ与垂直入射时第n条等剂量线深度的乘积。然而，这种方法忽略了散射的影响，对于大入射角的射束，在等剂量线位置上可能会导致显著误差。已经建立了一种半经验函数关系和一个等剂量线偏移因子表，利用它们可以轻松计算入射角在0度至60度之间的射束的任何等剂量线的深度。等剂量线偏移因子根据射束能量（6 - 22 MeV）和等剂量线（10% - 90%）制成表格，并且对于小于60度的角度，显示出相对独立于射束大小和入射角。已经使用平行板电离室和聚苯乙烯模体在Varian Clinac 2500直线加速器上进行了广泛测量。检查了电离室响应与射束角度的相关性，并确定聚苯乙烯模体的缩放因子等于1.00。
