Bjärngard B E, Vadash P, Zhu T
Department of Radiation Oncology, Roger Williams Cancer Center, Providence, Rhode Island 02908, USA.
Med Phys. 1995 Apr;22(4):465-8. doi: 10.1118/1.597477.
In an irradiation with a high-energy x-ray beam, the absorbed dose near the surface is the combined result of incident contaminating electrons and phantom-generated electrons. We describe an experimental method to characterize these processes under conditions of longitudinal electron disequilibrium but lateral equilibrium. The equilibrium dose at large depths is extrapolated back towards the surface and compared with measured doses. The extrapolation uses an expression that is based on Monte Carlo-calculated kerma values. The technique was applied to a 6-MV and a 25-MV x-ray beam. The dose from phantom-generated electrons increased exponentially with depth from zero at the surface. The dose from contaminating electrons decreased rapidly with depth with an attenuation coefficient that was approximately equal to the corresponding coefficient for the increase of dose from phantom-generated electrons. The surface dose from contaminating electrons increased linearly with the side of the square field at 6 MV but an error-function agreed better with the data at 25 MV.
在高能X射线束照射下，表面附近的吸收剂量是入射污染电子和模体产生电子共同作用的结果。我们描述了一种实验方法，用于在纵向电子不平衡但横向平衡的条件下表征这些过程。大深度处的平衡剂量向表面外推，并与测量剂量进行比较。外推使用基于蒙特卡洛计算比释动能值的表达式。该技术应用于6兆伏和25兆伏的X射线束。模体产生电子的剂量从表面的零值开始随深度呈指数增加。污染电子的剂量随深度迅速降低，其衰减系数近似等于模体产生电子剂量增加的相应系数。6兆伏时，污染电子的表面剂量随方形野边长呈线性增加，但误差函数与25兆伏时的数据拟合得更好。
