【摘要】 目的:模拟并分析医用直线加速器电子束在记录平面上的能谱分布,总结其分布特征。方法:基于蒙特卡罗方法的EGSnrc程序是医学物理领域中使用最广泛的模拟电子和光子输运过程的MC模拟程序,其计算精度已为大量实验所证实。本文用以EGSnrc为基础的BEAMnrc和BEAMdp程序对医用直线加速器在四种典型能量下的出射电子束能谱进行了模拟计算,在SSD平面记录所有初级和次级电子的能量和角分布,然后在该平面上选择不同大小的分析区域计算电子能谱。结果:首先在6 MeV、9 MeV、12 MeV、20 MeV四种不同能量,相同射野下选取相同大小分析区域,对其能谱进行分析,发现这些能谱都有一个主峰,并伴之以一个强度较小的次级峰,主峰半宽度几乎相同,而且都可以很好地用Pierson分布描述,但最可几能量和入射能量的比值随电子束能量的不同有很大差异;然后在12 MeV能量下选取不同大小分析区域对能谱进行分析,以考察能谱在分析平面内对位置依赖性。结论:在不同能量下,电子束能谱形状相似,可用Pierson分布描述;能谱几乎独立于所选择的分析区域,表明能谱的空间分布几乎是均匀的。更多还原

【Abstract】 Objective: To simulate the energy spectrum of the electron beam of a medical linear accelerator,and to characterize the energy spectrum in record plane.Method: The Monte Carlo(MC) code BEAMnrc and BEAMdp,which are based on a more generally purposed and widely used Monte Carlo code EGSnrc for the simulation of electron and photon transportation have been used to simulate the transport of electrons in a medical accelerator at four typical in radiation therapy source energies.The energy and angular distribution of the primary and secondary electron are recorded in SSD plane for further analysis in various analysis regions.Result: Generally,the energy spectrum is featured by a dominant peak companied by a minor peak.At the same size of field and analysis regions,it is observed that the half widths of the spectrum are almost the same for different source energies.All the spectrums can be well described by Pierson distributions.However,ratio of the most possible energy over the source energy could be quite different for different source energies.At fixed field size,the energy spectrum for 12MeV source energy is analyzed in different analysis regions to observe the dependence of the energy spectrum on the space.Conclusion: The electron beam spectrum is similar at different source energies and can be described by Pierson distribution.It is also found that the energy spectrum is almost independent to size of the analysis regions that suggest the energy spectrum is almost uniform to different locations in the analysis plane.更多还原