van der Zee W, Welleweerd J
Department of Radiotherapy, University Medical Center Utrecht, The Netherlands.
Med Phys. 1999 Sep;26(9):1883-92. doi: 10.1118/1.598694.
This study describes the results from a simulation of a 10 MV photon beam from a medical linac using the BEAM code. To check the quality of the generated photon beam, the characteristics of this beam (depth dose curve, cross profiles, and output factors) have been calculated and compared to measured data. By splitting up the radiation head in two parts, the target section and the collimator section calculation times were long, but acceptable when aiming at phase space files containing some 5 million particles. Given the number of particles evaluated, the accuracy of all data was around 2%. Analysis of the phase space files for different field size supports results from previous studies about contaminant particles and sources for scattered radiation for photon beams from medical linacs. The total scatter output factor Scp as well as the collimator scatter output factor Sc have been calculated within 2% of measurements. Also, the ratio between dose at a reference point for the full scatter situation and the no-scatter situation has been calculated correctly. All depth dose curves and cross profiles have also been calculated correctly, although with only moderate statistics. Improvements are possible by increasing the number of particles in the simulations (up to 50 million for the largest field size) at least 4-8 times, although calculation times will increase with the same factor. Nevertheless, the method proved itself as reliable. Still, the accuracy should be improved to 1% or better. This is necessary as we plan to use Monte Carlo simulations to benchmark three-dimensional radiotherapy planning systems. By increasing the number of particles in the phase space files and subsequently increasing the number of particles in each simulation, this 1% accuracy will be achieved. The easy way to increase the number of particles in a simulation by increasing the number of times phase space files, which were already recycled ten times, are reused from ten times (this study) to 40 times or more will not work, as it introduces artifacts, especially in the cross profiles.
本研究描述了使用BEAM代码对医用直线加速器产生的10 MV光子束进行模拟的结果。为检查所生成光子束的质量，已计算该束流的特性（深度剂量曲线、截面轮廓和输出因子）并与测量数据进行比较。通过将辐射头分为两部分，靶区和准直器区的计算时间较长，但对于包含约500万个粒子的相空间文件而言是可以接受的。考虑到所评估的粒子数量，所有数据的精度约为2%。对不同射野尺寸的相空间文件进行分析，支持了先前关于医用直线加速器光子束中污染粒子和散射辐射源的研究结果。总散射输出因子Scp以及准直器散射输出因子Sc的计算结果与测量值的偏差在2%以内。此外，全散射情况和无散射情况在参考点处剂量的比值也计算正确。所有深度剂量曲线和截面轮廓也计算正确，不过统计数据有限。通过将模拟中的粒子数量增加（最大射野尺寸下最多增加到5000万）至少4至8倍，有可能实现改进，尽管计算时间也会相应增加。然而，该方法证明是可靠的。不过，精度应提高到1%或更高。这是必要的，因为我们计划使用蒙特卡罗模拟来对标三维放射治疗计划系统。通过增加相空间文件中的粒子数量，进而增加每次模拟中的粒子数量，将实现1%的精度。通过增加相空间文件的重复使用次数（本研究中已重复使用十次），从十次增加到40次或更多来增加模拟中粒子数量的简单方法不可行，因为这会引入伪影，尤其是在截面轮廓中。
