Kato Hideki, Hayashi Naoki, Kuroki Ryohei, Adachi Yumiko, Suzuki Shizuma
Faculty of Radiological Technology, School of Health Sciences, Fujita Health University.
Nihon Hoshasen Gijutsu Gakkai Zasshi. 2013 Dec;69(12):1387-93. doi: 10.6009/jjrt.2013_jsrt_69.12.1387.
The energy spectra of high-energy electron beams used in radiotherapy are the most important data for evaluating absorbed doses and/or dose distributions in the body of a patient. However, it is impossible to measure the actual spectra of a high-energy electron beam. In this study, we suggest a method to presume the spectra of high-energy electron beams by use of the beta distribution model. The procedure of this method is as follows: (1) the spectrum of the high-energy electron beam was assumed to have a maximum energy Emax, and α, β parameters of the beta probability density function. (2) The percentage depth dose (PDD) based on the assumed spectrum was calculated by a Monte Carlo simulation. (3) The best matching energy spectrum was searched in comparison with the experimental PDD curves. Finally, the optimal energy spectrum of the electron beam was estimated after reiterating the process from (1) to (3). With our method, the measured PDD curves were optimally simulated following the experimental data. It appeared that the assumed spectra approximated well to the actual spectra. However, the error between the assumed and experimental data was observed in the region under the incident surface. We believe this was due to the influence of low-energy electrons scattered at installed collimators, etc. In order to simulate PDDs in this region accurately, a further correction process is required for a spectrum based on the beta distribution model.
放射治疗中使用的高能电子束能谱是评估患者体内吸收剂量和/或剂量分布的最重要数据。然而，测量高能电子束的实际能谱是不可能的。在本研究中，我们提出了一种利用贝塔分布模型推测高能电子束能谱的方法。该方法的步骤如下：（1）假设高能电子束的能谱具有最大能量Emax以及贝塔概率密度函数的α、β参数。（2）通过蒙特卡罗模拟计算基于假设能谱的百分深度剂量（PDD）。（3）与实验PDD曲线相比较，寻找最佳匹配能谱。最后，在重复从（1）到（3）的过程后，估计电子束的最佳能谱。使用我们的方法，测量得到的PDD曲线能按照实验数据进行最佳模拟。看起来假设能谱与实际能谱非常接近。然而，在入射表面下方区域观察到了假设数据与实验数据之间的误差。我们认为这是由于安装的准直器等散射的低能电子的影响。为了准确模拟该区域的PDD，基于贝塔分布模型的能谱需要进一步的校正过程。
