Sheikh-Bagheri D, Rogers D W, Ross C K, Seuntjens J P
Ionizing Radiation Standards, Institute for National Measurement Standards, National Research Council Canada, Ottawa.
Med Phys. 2000 Oct;27(10):2256-66. doi: 10.1118/1.1290714.
We have benchmarked photon beam simulations with the EGS4 user code BEAM [Rogers et al., Med. Phys. 22, 503-524 (1995)] by comparing calculated and measured relative ionization distributions in water from the 10 and 20 MV photon beams of the NRC linac. Unlike previous calculations, the incident electron energy is known independently to 1%, the entire extra-focal radiation is simulated, and electron contamination is accounted for. The full Monte Carlo simulation of the linac includes the electron exit window, target, flattening filter, monitor chambers, collimators, as well as the PMMA walls of the water phantom. Dose distributions are calculated using a modified version of the EGS4 user code DOSXYZ which additionally allows scoring of average energy and energy fluence in the phantom. Dose is converted to ionization by accounting for the (L/rho)water(air) variation in the phantom, calculated in an identical geometry for the realistic beams using a new EGS4 user code, SPRXYZ. The variation of (L/rho)water(air) with depth is a 1.25% correction at 10 MV and a 2% correction at 20 MV. At both energies, the calculated and the measured values of ionization on the central axis in the buildup region agree within 1% of maximum ionization relative to the ionization at 10 cm depth. The agreement is well within statistics elsewhere. The electron contamination contributes 0.35(+/- 0.02) to 1.37(+/- 0.03)% of the maximum dose in the buildup region at 10 MV and 0.26(+/- 0.03) to 3.14(+/- 0.07)% of the maximum dose at 20 MV. The penumbrae at 3 depths in each beam (in g/cm2), 1.99 (dmax, 10 MV only), 3.29 (dmax, 20 MV only), 9.79 and 19.79, agree with ionization chamber measurements to better than 1 mm. Possible causes for the discrepancy between calculations and measurements are analyzed and discussed in detail.
我们通过比较加拿大国家研究委员会（NRC）直线加速器10兆伏和20兆伏光子束在水中的计算相对电离分布与测量值，使用EGS4用户代码BEAM[罗杰斯等人，《医学物理》22，503 - 524（1995）]对光子束模拟进行了基准测试。与之前的计算不同，入射电子能量独立已知精度达1%，模拟了整个焦外辐射，并考虑了电子污染。直线加速器的全蒙特卡罗模拟包括电子出射窗、靶、均整滤过器、监测电离室、准直器以及水模体的聚甲基丙烯酸甲酯（PMMA）壁。剂量分布使用EGS4用户代码DOSXYZ的修改版本进行计算，该版本额外允许在模体中对平均能量和能量注量进行计分。通过考虑模体中（L/ρ）水（空气）的变化将剂量转换为电离，使用新的EGS4用户代码SPRXYZ在与实际射束相同的几何结构中进行计算。（L/ρ）水（空气）随深度的变化在10兆伏时为1.25%的校正，在20兆伏时为2%的校正。在两种能量下，在建成区中心轴上计算的和测量的电离值相对于10厘米深度处的电离，在最大电离的1%范围内一致。在其他位置，二者的一致性也完全在统计范围内。在10兆伏时，电子污染对建成区最大剂量的贡献为0.35（±0.02）%至1.37（±0.03）%，在20兆伏时为0.26（±0.03）%至3.14（±0.07）%。每束射束在3个深度处（以克/平方厘米为单位）的半值层，分别为1.99（仅10兆伏的dmax）、3.29（仅20兆伏的dmax）、9.79和19.79，与电离室测量值的一致性优于1毫米。详细分析和讨论了计算值与测量值之间差异的可能原因。
