医用直线加速器不同靶材料对X射线能谱的影响研究

Study on the effect of different target materials of accelerator on the quality of X rays

目的:研究无均整过滤器(FFF)模式下不同靶材料对加速器能谱结构的影响,以及该模式下是否有必要切换靶材料以满足不同的临床治疗需求。方法:以基于蒙特卡罗算法(统计试验法)的EGSnrc程序用于电离辐射传输模拟,使用BEAMnrc建立VarianTrueBeam系列加速器机头结构,并设置不同材料和不同厚度的靶,使用BEAMdp调用其计算生成的相空间文件进行能谱与能量注量的结果分析比较。结果:在靶材料中厚度为0.2 cm的金靶在能谱结构表现较钽靶、钨靶更为优异,高原子序数材料光子能量注量差异不大,低原子序数材料铜靶在能谱与能量注量都不能满足使用要求。金靶在<2 cm厚度时具有很高的电子污染,低能光子份额高达20%,非平坦度高于80%,但较薄材料具有更高的能量份额与更低的热量损失,在常规模式较为适用。结论:FFF模式较常规模式提高了射野范围内光子的产生效率,高原子序数材料更适合作为靶材料,在常规模式切换到FFF模式时可以考虑同时切换靶材料以提高轫致辐射产生效率。

Objective:To explore the influence of different target materials on the energy spectrum structure of the accelerator under flattening filter free(FFF)mode,and whether it is necessary to switch target materials in this mode so as to meet the requirement of different clinical treatment.Methods:The EGSnrc program based on the Monte Carlo algorithm(statistical test method)was used for transmission simulation of ionizing radiation,and the head structure of the Varian TrueBeam series accelerator was established by using BEAmnrc,and targets with different materials and different thicknesses were set.BEAMdp was applied to use phases space file which was calculated and produced by it for analyzing and comparing the results of energy spectrum and energy fluence.Results:0.2 cm Au target has better performance in the appearance of energy spectrum structure than TA target and W target.The differences were less in energy fluence of photon of materials with high atomic number,and the energy spectrum and energy fluence of Cu target of materials with low atomic number couldn’t meet the operating requirement.Au target has very high electronic pollution when its thickness was less than 2 cm,and the percentage of low energy photon reach to 20%and the nonflatness degree higher than 80%at this condition.The thinner material is relatively fit for routine mode because it has higher energy percentage and lower thermal loss.Conclusion:Compared with the routine mode,the FFF mode improves the efficiency of photon generation in the field range.The material with high atomic number is more suitable as the target material.When the conventional mode is switched to the FFF mode,the operation of switching the target material at the same time for improving the generation efficiency of bremsstrahlung should be considered.