Ferri A, Ruggeri R
Servizio di Fisica Sanitaria, Policlinico S. Orsola-Malpighi, Azienda Ospedaliera di Bologna.
Radiol Med. 1997 Oct;94(4):385-7.
In the dose calculation at an arbitrary point in a water-equivalent homogeneous medium irradiated by an X-ray megavoltage wedged beam (w), several treatment planning systems (TPS) based on the Bently-Milan data processing procedure use algorithms which do not distinguish between wedged OF(s)w and open OF(s)o output factor or between field size dependent WF(s) and not WF wedge factor. This is equivalent to neglecting the difference between open (o) and wedged (w) fields both in relation to back-scattered radiation from secondary collimators to the beam monitor chamber and to scattered radiation generated by the water-phantom at the depth of measurement. The output factor can be expressed by: OF(s) = Sc(s)* Sp(s) where Sc(s) is the collimator scatter factor and Sp(s) is the phantom scatter factor. The approximation between wedged OF(s)w and open OF(s)o is thus equivalent to ignoring the fact that the insertion of the wedge in an open field modifies the back-scattered radiation from secondary collimators to the beam monitor chamber and the scattered radiation in the phantom. The lack of wedge factor's field size dependence WF(s) or the equivalent lack of wedged field output factor OF(s)w in the beam data installed in TPS is in appreciable source of systematic error in the calculation of the absorbed dose. This systematic uncertainty can be evaluated by measuring the wedge factor's field size dependence. In our study of WF(s) for 25-MV X-rays generated by our Linac "Saturne-43" (by GE), the relative wedge factor variation with respect to the reference value WF(so) ranges from (-.7%) for the smallest wedged field size (5 x 5) cm2 to (+2.6%) for the largest wedged field size (20 x 20) cm2. It is clear that the same relative variation reproduces itself identically as a systematic uncertainty in the calculation of the absorbed dose.
在由兆伏级X射线楔形束（w）照射的水等效均匀介质中任意一点进行剂量计算时，基于本特利 - 米兰数据处理程序的几种治疗计划系统（TPS）使用的算法无法区分楔形野OF(s)w和开放野OF(s)o输出因子，也无法区分与射野大小相关的WF(s)和非WF楔形因子。这等同于忽略了开放野（o）和楔形野（w）在从二级准直器到射束监测电离室的反向散射辐射以及在测量深度处水模体产生的散射辐射方面的差异。输出因子可表示为：OF(s) = Sc(s)* Sp(s)，其中Sc(s)是准直器散射因子，Sp(s)是模体散射因子。因此，楔形野OF(s)w和开放野OF(s)o之间的近似等同于忽略了在开放野中插入楔形物会改变从二级准直器到射束监测电离室的反向散射辐射以及模体中的散射辐射这一事实。TPS中安装的射束数据缺乏楔形因子对射野大小的依赖性WF(s)，或者等效地缺乏楔形野输出因子OF(s)w，这是吸收剂量计算中系统误差的一个重要来源。这种系统不确定性可以通过测量楔形因子对射野大小的依赖性来评估。在我们对由我们的直线加速器“Saturne - 43”（通用电气公司生产）产生的25兆伏X射线的WF(s)的研究中，相对于参考值WF(so)的相对楔形因子变化范围从最小楔形野大小（5×5）平方厘米的（ - 0.7%）到最大楔形野大小（20×20）平方厘米的（ + 2.6%）。很明显，相同的相对变化在吸收剂量计算中作为系统不确定性会完全重现。
