Ahnesjö A, Weber L, Nilsson P
Helax AB, Uppsala, Sweden.
Med Phys. 1995 Nov;22(11 Pt 1):1711-20. doi: 10.1118/1.597534.
The development of treatment planning methods in radiation therapy requires dose calculation methods that are both accurate and general enough to provide a dose per unit monitor setting for a broad variety of fields and beam modifiers. The purpose of this work was to develop models for calculation of scatter and transmission for photon beam attenuators such as compensating filters, wedges, and block trays. The attenuation of the beam is calculated using a spectrum of the beam, and a correction factor based on attenuation measurements. Small angle coherent scatter and electron binding effects on scattering cross sections are considered by use of a correction factor. Quality changes in beam penetrability and energy fluence to dose conversion are modeled by use of the calculated primary beam spectrum after passage through the attenuator. The beam spectra are derived by the depth dose effective method, i.e., by minimizing the difference between measured and calculated depth dose distributions, where the calculated distributions are derived by superposing data from a database for monoenergetic photons. The attenuator scatter is integrated over the area viewed from the calculation point of view using first scatter theory. Calculations are simplified by replacing the energy and angular-dependent cross-section formulas with the forward scatter constant r2(0) and a set of parametrized correction functions. The set of corrections include functions for the Compton energy loss, scatter attenuation, and secondary bremsstrahlung production. The effect of charged particle contamination is bypassed by avoiding use of dmax for absolute dose calibrations. The results of the model are compared with scatter measurements in air for copper and lead filters and with dose to a water phantom for lead filters for 4 and 18 MV. For attenuated beams, downstream of the buildup region, the calculated results agree with measurements on the 1.5% level. The accuracy was slightly less in situations where the scatter component is very large, as for very large fields with very short filter to detector distances. The implementation of the model into treatment planning systems is discussed.
放射治疗中治疗计划方法的发展需要剂量计算方法，这种方法既要准确又要通用，以便能为各种射野和射束修正器提供每单位监测器设置的剂量。这项工作的目的是开发用于计算光子束衰减器（如补偿滤过器、楔形板和挡块托盘）散射和透射的模型。使用射束的能谱以及基于衰减测量的校正因子来计算射束的衰减。通过使用校正因子来考虑小角度相干散射和电子束缚对散射截面的影响。通过使用计算得到的经过衰减器后的原射线能谱，对射束穿透性和能量注量到剂量转换的质量变化进行建模。射束能谱通过深度剂量有效方法得出，即通过最小化测量和计算的深度剂量分布之间的差异得出，其中计算分布是通过叠加来自单能光子数据库的数据得出。使用首次散射理论，在从计算角度观察的区域上对衰减器散射进行积分。通过用前向散射常数r2(0)和一组参数化校正函数代替能量和角度相关的截面公式来简化计算。这组校正包括康普顿能量损失、散射衰减和二次轫致辐射产生的函数。通过避免使用dmax进行绝对剂量校准来绕过带电粒子污染的影响。将该模型的结果与铜和铅滤过器在空气中的散射测量结果以及4和18MV铅滤过器对水模体的剂量测量结果进行比较。对于衰减后的射束，在建成区下游，计算结果与测量结果在1.5%的水平上相符。在散射分量非常大的情况下，如对于滤过器到探测器距离非常短的非常大的射野，精度略低。讨论了将该模型应用于治疗计划系统的情况。
