Henry Ford Health System, Detroit, Michigan 48202, USA.
Med Phys. 2009 Dec;36(12):5451-66. doi: 10.1118/1.3253300.
In this work, an investigation of efficiency enhancing methods and cross-section data in the BEAMnrc Monte Carlo (MC) code system is presented. Additionally, BEAMnrc was compared with VMC++, another special-purpose MC code system that has recently been enhanced for the simulation of the entire treatment head. BEAMnrc and VMC++ were used to simulate a 6 MV photon beam from a Siemens Primus linear accelerator (linac) and phase space (PHSP) files were generated at 100 cm source-to-surface distance for the 10 x 10 and 40 x 40 cm2 field sizes. The BEAMnrc parameters/techniques under investigation were grouped by (i) photon and bremsstrahlung cross sections, (ii) approximate efficiency improving techniques (AEITs), (iii) variance reduction techniques (VRTs), and (iv) a VRT (bremsstrahlung photon splitting) in combination with an AEIT (charged particle range rejection). The BEAMnrc PHSP file obtained without the efficiency enhancing techniques under study or, when not possible, with their default values (e.g., EXACT algorithm for the boundary crossing algorithm) and with the default cross-section data (PEGS4 and Bethe-Heitler) was used as the "base line" for accuracy verification of the PHSP files generated from the different groups described previously. Subsequently, a selection of the PHSP files was used as input for DOSXYZnrc-based water phantom dose calculations, which were verified against measurements. The performance of the different VRTs and AEITs available in BEAMnrc and of VMC++ was specified by the relative efficiency, i.e., by the efficiency of the MC simulation relative to that of the BEAMnrc base-line calculation. The highest relative efficiencies were approximately 935 (approximately 111 min on a single 2.6 GHz processor) and approximately 200 (approximately 45 min on a single processor) for the 10 x 10 field size with 50 million histories and 40 x 40 cm2 field size with 100 million histories, respectively, using the VRT directional bremsstrahlung splitting (DBS) with no electron splitting. When DBS was used with electron splitting and combined with augmented charged particle range rejection, a technique recently introduced in BEAMnrc, relative efficiencies were approximately 420 (approximately 253 min on a single processor) and approximately 175 (approximately 58 min on a single processor) for the 10 x 10 and 40 x 40 cm2 field sizes, respectively. Calculations of the Siemens Primus treatment head with VMC++ produced relative efficiencies of approximately 1400 (approximately 6 min on a single processor) and approximately 60 (approximately 4 min on a single processor) for the 10 x 10 and 40 x 40 cm2 field sizes, respectively. BEAMnrc PHSP calculations with DBS alone or DBS in combination with charged particle range rejection were more efficient than the other efficiency enhancing techniques used. Using VMC++, accurate simulations of the entire linac treatment head were performed within minutes on a single processor. Noteworthy differences (+/- 1%-3%) in the mean energy, planar fluence, and angular and spectral distributions were observed with the NIST bremsstrahlung cross sections compared with those of Bethe-Heitler (BEAMnrc default bremsstrahlung cross section). However, MC calculated dose distributions in water phantoms (using combinations of VRTs/AEITs and cross-section data) agreed within 2% of measurements. Furthermore, MC calculated dose distributions in a simulated water/air/water phantom, using NIST cross sections, were within 2% agreement with the BEAMnrc Bethe-Heitler default case.
在这项工作中，我们研究了 BEAMnrc 蒙特卡罗（MC）代码系统中的增效方法和截面数据。此外，我们还将 BEAMnrc 与 VMC++进行了比较，后者是一种最近为模拟整个治疗头而增强的专用 MC 代码系统。我们使用 BEAMnrc 和 VMC++模拟了来自 Siemens Primus 直线加速器（linac）的 6 MV 光子束，并在 100 cm 源皮距处为 10 x 10 和 40 x 40 cm2 射野大小生成了 PHSP 文件。研究中涉及的 BEAMnrc 参数/技术分为以下几类：（i）光子和韧致辐射截面，（ii）近似效率提高技术（AEIT），（iii）方差减少技术（VRT），以及（iv）与 AEIT（带电粒子射程排斥）相结合的 VRT（韧致辐射光子分裂）。没有研究中的增效技术或不可能使用默认值（例如，边界穿越算法的 EXACT 算法）生成的 BEAMnrc PHSP 文件，以及使用默认截面数据（PEGS4 和 Bethe-Heitler）生成的 PHSP 文件被用作之前描述的不同组生成的 PHSP 文件的准确性验证的“基准”。随后，选择了一些 PHSP 文件作为基于 DOSXYZnrc 的水模剂量计算的输入，并与测量结果进行了验证。BEAMnrc 和 VMC++中可用的不同 VRT 和 AEIT 的性能由相对效率指定，即 MC 模拟的效率相对于 BEAMnrc 基准计算的效率。对于 10 x 10 场大小，当使用 5000 万次历史记录和 40 x 40 cm2 场大小时，最高相对效率分别约为 935（大约在单个 2.6 GHz 处理器上运行 111 分钟）和约 200（大约在单个处理器上运行 45 分钟），而对于不使用电子分裂的定向韧致辐射分裂（DBS）VRT。当 DBS 与电子分裂一起使用并与增强的带电粒子射程排斥相结合时，这是 BEAMnrc 中最近引入的技术，对于 10 x 10 和 40 x 40 cm2 射野大小，相对效率分别约为 420（大约在单个处理器上运行 253 分钟）和约 175（大约在单个处理器上运行 58 分钟）。使用 VMC++计算 Siemens Primus 治疗头的相对效率分别约为 1400（大约在单个处理器上运行 6 分钟）和约 60（大约在单个处理器上运行 4 分钟）对于 10 x 10 和 40 x 40 cm2 射野大小。单独使用 DBS 或 DBS 与带电粒子射程排斥相结合的 BEAMnrc PHSP 计算比使用的其他增效技术更有效。使用 VMC++，可以在单个处理器上在几分钟内完成整个直线加速器治疗头的精确模拟。与 Bethe-Heitler（BEAMnrc 默认韧致辐射截面）相比，使用 NIST 韧致辐射截面观察到平均能量、平面通量、角度和光谱分布的显著差异（+/- 1%-3%）。然而，在水模体中（使用 VRT/AEIT 和截面数据的组合）计算的 MC 剂量分布与测量结果的差异在 2%以内。此外，使用 NIST 截面在模拟的水/空气/水模体中计算的 MC 剂量分布与 BEAMnrc Bethe-Heitler 默认情况的差异在 2%以内。
