Cygler J E, Daskalov G M, Chan G H, Ding G X
Medical Physics Department, Ottawa Regional Cancer Centre, 503 Smyth Road, Ottawa, Ontario K1H 1C4, Canada.
Med Phys. 2004 Jan;31(1):142-53. doi: 10.1118/1.1633105.
The purpose of this study is to perform a clinical evaluation of the first commercial (MDS Nordion, now Nucletron) treatment planning system for electron beams incorporating Monte Carlo dose calculation module. This software implements Kawrakow's VMC++ voxel-based Monte Carlo calculation algorithm. The accuracy of the dose distribution calculations is evaluated by direct comparisons with extensive sets of measured data in homogeneous and heterogeneous phantoms at different source-to-surface distances (SSDs) and gantry angles. We also verify the accuracy of the Monte Carlo module for monitor unit calculations in comparison with independent hand calculations for homogeneous water phantom at two different SSDs. All electron beams in the range 6-20 MeV are from a Siemens KD-2 linear accelerator. We used 10,000 or 50,000 histories/cm2 in our Monte Carlo calculations, which led to about 2.5% and 1% relative standard error of the mean of the calculated dose. The dose calculation time depends on the number of histories, the number of voxels used to map the patient anatomy, the field size, and the beam energy. The typical run time of the Monte Carlo calculations (10,000 histories/cm2) is 1.02 min on a 2.2 GHz Pentium 4 Xeon computer for a 9 MeV beam, 10 x 10 cm2 field size, incident on the phantom 15 x 15 x 10 cm3 consisting of 31 CT slices and voxels size of 3 x 3 x 3 mm3 (total of 486,720 voxels). We find good agreement (discrepancies smaller than 5%) for most of the tested dose distributions. We also find excellent agreement (discrepancies of 2.5% or less) for the monitor unit calculations relative to the independent manual calculations. The accuracy of monitor unit calculations does not depend on the SSD used, which allows the use of one virtual machine for each beam energy for all arbitrary SSDs. In some cases the test results are found to be sensitive to the voxel size applied such that bigger systematic errors (>5%) occur when large voxel sizes interfere with the extensions of heterogeneities or dose gradients because of differences between the experimental and calculated geometries. Therefore, user control over voxelization is important for high accuracy electron dose calculations.
本研究的目的是对首个商业化的(MDS Nordion,现Nucletron)包含蒙特卡罗剂量计算模块的电子束治疗计划系统进行临床评估。该软件实现了Kawrakow基于体素的VMC++蒙特卡罗计算算法。通过与在不同源皮距(SSD)和机架角度下的均匀和非均匀体模中的大量测量数据进行直接比较,评估剂量分布计算的准确性。我们还将蒙特卡罗模块在监测单位计算方面的准确性与在两个不同SSD下对均匀水体模进行的独立手工计算进行了比较。6 - 20 MeV范围内的所有电子束均来自西门子KD - 2直线加速器。在蒙特卡罗计算中,我们使用了10,000或50,000历史记录/cm²,这导致计算剂量平均值的相对标准误差约为2.5%和1%。剂量计算时间取决于历史记录数量、用于映射患者解剖结构的体素数量、射野大小和束能量。在一台2.2 GHz奔腾4至强计算机上,对于9 MeV束、10×10 cm²射野大小、入射到由31个CT切片组成的15×15×10 cm³体模且体素大小为3×3×3 mm³(总共486,720个体素)的情况,蒙特卡罗计算(10,000历史记录/cm²)的典型运行时间为1.02分钟。我们发现大多数测试的剂量分布具有良好的一致性(差异小于5%)。我们还发现监测单位计算相对于独立手工计算具有极佳的一致性(差异为2.5%或更小)。监测单位计算的准确性不取决于所使用的SSD,这使得对于所有任意SSD,每种束能量都可以使用一个虚拟机。在某些情况下,发现测试结果对所应用的体素大小敏感,以至于当大的体素大小由于实验和计算几何形状之间的差异而干扰非均匀性或剂量梯度的扩展时,会出现更大的系统误差(>5%)。因此,用户对体素化的控制对于高精度电子剂量计算很重要。