Azimuthal particle redistribution for the reduction of latent phase-space variance in Monte Carlo simulations
Azimuthal particle redistribution for the reduction of latent phase-space variance in Monte Carlo simulations
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DOI：
10.1088/0031-9155/52/14/021
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发表时间：
2007-07-21
影响因子：
3.5
通讯作者：
Beckham, W. A.
中科院分区：
文献类型：
--
作者：
Bush, K.;Zavgorodni, S. F.;Beckham, W. A.
众所周知，在蒙特卡罗模拟中使用相空间引入了不能通过使用标准粒子回收技术来抑制的基线水平的方差。该方差（Sempau等人称为潜在相空间方差）可能是实现准确、低不确定性剂量评分结果的重要限制因素，尤其是在体模表面附近。一个BEAMnrc组件模块（MCTWIST）已被开发，以减少存在的潜在的变化，在相空间为基础的蒙特卡罗模拟通过实施方位角粒子再分布（APR）。对于相空间粒子的每次循环使用，施加围绕束的中心轴的随机旋转，有效地利用粒子注量的圆柱对称性，从而提供源的更准确的表示。MCTWIST模块的独特之处在于，实际上没有将物理组件添加到加速器几何结构中。通过直接转换BEAMnrc/EGSnrc粒子传输之外的粒子特性进行射束修改。使用MCTWIST，我们已经证明了减少潜在的相空间方差超过20倍，为10 × 10厘米（2）的领域，相比标准的相空间粒子回收技术。潜在方差的减少使得能够实现显著更平滑的水中剂量分布。本文概述了使用MCTWIST在蒙特卡洛模拟和量化的潜在方差减少首次利用圆柱相空间对称性。
It is well known that the use of a phase space in Monte Carlo simulation introduces a baseline level of variance that cannot be suppressed through the use of standard particle recycling techniques. This variance (termed latent phase-space variance by Sempau et al) can be a significant limiting factor in achieving accurate, low-uncertainty dose scoring results, especially near the surface of a phantom. A BEAMnrc component module (MCTWIST) has been developed to reduce the presence of latent variance in phase-space-based Monte Carlo simulations by implementing azimuthal particle redistribution (APR). For each recycled use of a phase-space particle a random rotation about the beam's central axis is applied, effectively utilizing cylindrical symmetry of the particle fluence and therefore providing a more accurate representation of the source. The MCTWIST module is unique in that no physical component is actually added to the accelerator geometry. Beam modifications are made by directly transforming particle characteristics outside of BEAMnrc/EGSnrc particle transport. Using MCTWIST, we have demonstrated a reduction in latent phase-space variance by more than a factor of 20, for a 10 x 10 cm(2) field, when compared to standard phase-space particle recycling techniques. The reduction in latent variance has enabled the achievement of dramatically smoother in-water dose profiles. This paper outlines the use of MCTWIST in Monte Carlo simulation and quantifies for the first time the latent variance reduction resulting from exploiting cylindrical phase-space symmetry.