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Fast, accurate photon beam accelerator modeling using BEAMnrc : A systematic investigation of efficiency enhancing methods and cross-section data

机译:使用BEAMnrc进行快速,准确的光子束加速器建模:效率提高方法和横截面数据的系统研究

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摘要

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\u2002MV photon beam from a Siemens Primus linear accelerator (linac) and phase space (PHSP) files were generated at 100\u2002cm source-to-surface distance for the 10\ud710 and 40\ud740\u2002cm2 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\u2013Heitler) was used as the \u201cbase line\u201d 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 \u223c935 (\u223c111\u2002min on a single 2.6\u2002GHz processor) and \u223c200 (\u223c45\u2002min on a single processor) for the 10\ud710 field size with 50 million histories and 40\ud740\u2002cm2 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 \u223c420 (\u223c253\u2002min on a single processor) and \u223c175 (\u223c58\u2002min on a single processor) for the 10\ud710 and 40\ud740\u2002cm2 field sizes, respectively. Calculations of the Siemens Primus treatment head with VMC++ produced relative efficiencies of \u223c1400 (\u223c6\u2002min on a single processor) and \u223c60 (\u223c4\u2002min on a single processor) for the 10\ud710 and 40\ud740\u2002cm2 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 (\ub11%\u20133%) in the mean energy, planar fluence, and angular and spectral distributions were observed with the NIST bremsstrahlung cross sections compared with those of Bethe\u2013Heitler (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\u2013Heitler default case.
机译:在这项工作中,对BEAMnrc Monte Carlo(MC)代码系统中的效率增强方法和横截面数据进行了研究。此外,还将BEAMnrc与VMC ++(另一种专用MC代码系统)进行了比较,VMC ++最近针对模拟整个治疗头进行了增强。使用BEAMnrc和VMC ++模拟来自Siemens Primus线性加速器(linac)的6 \ u2002MV光子束,并以10 \ ud710和40 \ ud740的源到表面距离在100 \ u2002cm处生成相空间(PHSP)文件\ u2002cm2字段大小。所研究的BEAMnrc参数/技术按(i)光子和致发光横截面,(ii)近似效率改进技术(AEIT),(iii)降低方差技术(VRT)和(iv)VRT(bre致发光光子分裂)分组。 )与AEIT(带电粒子范围抑制)结合使用。使用没有研究中的效率增强技术而获得的BEAMnrc PHSP文件,或者在不可能的情况下使用其默认值(例如,边界越界算法的EXACT算法)和默认横截面数据(PEGS4和Bethe \ u2013Heitler)获得的文件作为\ u201cbase行\ u201d的信息,以验证从上述不同组生成的PHSP文件的准确性。随后,将选择的PHSP文件用作基于DOSXYZnrc的水体模剂量计算的输入,并根据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(即MC模拟相对于BEAMnrc基线计算的效率)指定。对于具有5000万历史和40 \ ud740 \ u2002cm2的10 \ ud710字段大小,最高相对效率是\ u223c935(在单个2.6 \ u2002GHz处理器上为\ u223c111 \ u2002min)和\ u223c200(在单个处理器上为\ u223c45 \ u2002min)使用无电子分裂的VRT定向致裂分裂(DBS),分别具有1亿个历史的场大小。当DBS与电子分裂一起使用,并与增加的带电粒子范围抑制结合使用时,BEAMnrc中最近引入了一项技术,相对效率为\ u223c420(在单个处理器上为\ u223c253 \ u2002min)和\ u223c175(在单个处理器上为\ u223c58 \ u2002min) )分别用于10 \ ud710和40 \ ud740 \ u2002cm2字段大小。对于10 \ ud710和40 \ ud740 \ u2002cm2字段,使用VMC ++对Siemens Primus处理头的计算得出的相对效率为\ u223c1400(在单个处理器上为\ u223c6 \ u2002min)和\ u223c60(在单个处理器上为\ u223c4 \ u2002min)大小。单独使用DBS或结合带电粒子范围抑制的DBS进行BEAMnrc PHSP计算比使用的其他效率增强技术更有效。使用VMC ++,可以在几分钟内在单个处理器上对整个直线加速器治疗头进行精确的仿真。与Bethe \ u2013Heitler(BEAMnrc默认bremsstrahlung横截面)相比,NIST bremsstrahlung横截面的平均能量,平面通量以及角度和光谱分布存在显着差异(\ ub11%\ u20133%)。但是,MC计算出的水体模型中的剂量分布(使用VRT / AEIT和横截面数据的组合)在测量值的2%之内一致。此外,MC使用NIST横截面在模拟的水/空气/水模型中计算出的剂量分布与BEAMnrc Bethe \ u2013Heitler默认情况在2%的范围内。

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