Mora G M, Maio A, Rogers D W
CFNUL and FCUL, Universidade de Lisboa, Portugal.
Med Phys. 1999 Nov;26(11):2494-502. doi: 10.1118/1.598770.
The BEAM Monte Carlo code is used to simulate the 60Co beam from an Eldorado 6 radiotherapy unit and to calculate the relative air-kerma output factors as a function of field size. The unit is realistically modeled, including source capsule, housing and collimator assembly. The calculated relative air-kerma output factors at SSD=80.5 cm agree to within 0.1% with measured values. It is shown that the variation of the output factor is almost entirely due to scattered photons from the fixed and adjustable collimators and there is no effect of shadowing primary photons. The influence of the geometry of the collimation system on the photon spectra on-axis is shown to be small but finite. The calculated buildup region of a depth-dose curve in a water phantom irradiated by a narrow and a broad 60Co beam is shown to agree with experimental data at the 2% to 3% level. Unlike previous calculations, the results accurately predict the effects of electron contamination from the surface to dose maximum. The variation of electron contamination with field size is also presented, as are spectra as a function of field size.
BEAM蒙特卡罗代码用于模拟来自Eldorado 6放射治疗单元的60Co束流，并计算作为射野大小函数的相对空气比释动能输出因子。对该单元进行了逼真的建模，包括源容器、外壳和准直器组件。在源皮距（SSD）=80.5 cm时计算得到的相对空气比释动能输出因子与测量值的偏差在0.1%以内。结果表明，输出因子的变化几乎完全归因于来自固定和可调准直器的散射光子，而初级光子的遮蔽没有影响。准直系统的几何结构对轴上光子能谱的影响虽小但有限。计算得到的窄束和宽束60Co束流照射水模体时深度剂量曲线的积累区与实验数据在2%至3%的水平上相符。与之前的计算不同，该结果准确预测了从表面到剂量最大值的电子污染效应。还给出了电子污染随射野大小的变化以及作为射野大小函数的能谱。
