Fix Michael K, Keall Paul J, Siebers Jeffrey V
Department of Radiation Oncology, Virginia Commonwealth University, PO Box 980058, Richmond, Virginia 23298, USA.
Med Phys. 2005 Apr;32(4):1164-75. doi: 10.1118/1.1884385.
One limitation to the widespread implementation of Monte Carlo (MC) patient dose-calculation algorithms for radiotherapy is the lack of a general and accurate source model of the accelerator radiation source. Our aim in this work is to investigate the sensitivity of the photon-beam subsource distributions in a MC source model (with target, primary collimator, and flattening filter photon subsources and an electron subsource) for 6- and 18-MV photon beams when the energy and radial distributions of initial electrons striking a linac target change. For this purpose, phase-space data (PSD) was calculated for various mean electron energies striking the target, various normally distributed electron energy spread, and various normally distributed electron radial intensity distributions. All PSD was analyzed in terms of energy, fluence, and energy fluence distributions, which were compared between the different parameter sets. The energy spread was found to have a negligible influence on the subsource distributions. The mean energy and radial intensity significantly changed the target subsource distribution shapes and intensities. For the primary collimator and flattening filter subsources, the distribution shapes of the fluence and energy fluence changed little for different mean electron energies striking the target, however, their relative intensity compared with the target subsource change, which can be accounted for by a scaling factor. This study indicates that adjustments to MC source models can likely be limited to adjusting the target subsource in conjunction with scaling the relative intensity and energy spectrum of the primary collimator, flattening filter, and electron subsources when the energy and radial distributions of the initial electron-beam change.
蒙特卡罗（MC）患者放疗剂量计算算法广泛应用的一个限制是缺乏通用且准确的加速器辐射源模型。本研究的目的是在直线加速器靶材上初始电子的能量和径向分布发生变化时，研究MC源模型（包含靶材、初级准直器、均整滤过器光子子源和电子子源）中6兆伏和18兆伏光子束的光子束子源分布的敏感性。为此，针对撞击靶材的各种平均电子能量、各种呈正态分布的电子能量展宽以及各种呈正态分布的电子径向强度分布，计算了相空间数据（PSD）。所有PSD均根据能量、注量和能量注量分布进行分析，并在不同参数集之间进行比较。结果发现，能量展宽对子源分布的影响可忽略不计。平均能量和径向强度显著改变了靶材子源的分布形状和强度。对于初级准直器和均整滤过器子源，撞击靶材的不同平均电子能量下，注量和能量注量的分布形状变化不大，然而，它们与靶材子源相比的相对强度发生了变化，这可以用一个比例因子来解释。本研究表明，当初始电子束的能量和径向分布发生变化时，对MC源模型的调整可能仅限于调整靶材子源，并按比例缩放初级准直器、均整滤过器和电子子源的相对强度及能谱。
