Wang L, Chui C S, Lovelock M
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
Med Phys. 1998 Jun;25(6):867-78. doi: 10.1118/1.598262.
A patient-specific, CT-based, Monte Carlo dose-calculation method for photon beams has been developed to correctly account for inhomogeneity in the patient. The method employs the EGS4 system to sample the interaction of radiation in the medium. CT images are used to describe the patient geometry and to determine the density and atomic number in each voxel. The user code (MCPAT) provides the data describing the incident beams, and performs geometry checking and energy scoring in patient CT images. Several variance reduction techniques have been implemented to improve the computation efficiency. The method was verified with measured data and other calculations, both in homogeneous and inhomogeneous media. The method was also applied to a lung treatment, where significant differences in dose distributions, especially in the low-density region, were observed when compared with the results using an equivalent pathlength method. Comparison of the DVHs showed that the Monte Carlo calculated plan predicted an underdose of nearly 20% to the target, while the maximum doses to the cord and the heart were increased by 25% and 33%, respectively. These results suggested that the Monte Carlo method may have an impact on treatment designs, and also that it can be used as a benchmark to assess the accuracy of other dose calculation algorithms. The computation time for the lung case employing five 15-MV wedged beams, with an approximate field size of 13 X 13 cm and the dose grid size of 0.375 cm, was less than 14 h on a 175-MHz computer with a standard deviation of 1.5% in the high-dose region.
已开发出一种基于患者CT的光子束蒙特卡罗剂量计算方法，以正确考虑患者体内的不均匀性。该方法采用EGS4系统对介质中的辐射相互作用进行采样。CT图像用于描述患者的几何形状，并确定每个体素中的密度和原子序数。用户代码（MCPAT）提供描述入射束的数据，并在患者CT图像中进行几何检查和能量计分。已实施了几种方差减少技术以提高计算效率。该方法在均匀和非均匀介质中均通过测量数据和其他计算进行了验证。该方法还应用于肺部治疗，与使用等效路径长度方法的结果相比，观察到剂量分布存在显著差异，尤其是在低密度区域。DVH的比较表明，蒙特卡罗计算的计划预测靶区剂量不足近20%，而脊髓和心脏的最大剂量分别增加了25%和33%。这些结果表明，蒙特卡罗方法可能会对治疗设计产生影响，并且还可以用作评估其他剂量计算算法准确性的基准。在一台175-MHz的计算机上，对于肺部病例，使用五条15-MV楔形束，近似射野尺寸为13×13 cm，剂量网格尺寸为0.375 cm，计算时间少于14小时，高剂量区域的标准偏差为1.5%。
