Toossi Mohammad Taghi Bahreyni, Abdollahi Malihe, Ghorbani Mahdi
Medical Physics Research Center, Medical Physics Department, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran.
Rep Pract Oncol Radiother. 2010 Dec 21;16(1):10-3. doi: 10.1016/j.rpor.2010.11.005. eCollection 2010.
As a routine method for stepping source simulation, a Monte Carlo program is run according to the number of steps and then the summation of dose from each run is taken to obtain total dose distribution. This method is time consuming.
As an alternative method, a matrix shift based technique was applied to simulate a stepping source for brachytherapy.
The stepping source of GZP6 brachytherapy unit was simulated. In a matrix shift method, it is assumed that a radiation source is stationary and instead the data matrix is shifted based on the number of steps. In this study, by running MCNPX program for one point and calculation of the dose matrix using the matrix shift method, the isodose curves for the esophageal cancer tumor lengths of 4 and 6 cm were obtained and compared with the isodose curves obtained by running MCNPX programs in each step position separately (15 and 23 steps for esophageal cancer tumor lengths of 4 and 6 cm, respectively).
The difference between the two dose matrixes for the stepping and matrix shift methods based on the average dose differences are 3.85 × 10(-4) Gy and 5.19 × 10(-4) Gy for treatment length of 4 cm and 6 cm, respectively. Dose differences are insignificant and these two methods are equally valid.
The matrix shift method presented in this study can be used for calculation of dose distribution for a brachytherapy stepping source as a quicker tool compared to other routine Monte Carlo based methods.
作为步进源模拟的常规方法，根据步数运行蒙特卡罗程序，然后对每次运行的剂量求和以获得总剂量分布。这种方法很耗时。
作为一种替代方法，应用基于矩阵移位的技术来模拟近距离放射治疗的步进源。
模拟了GZP6近距离放射治疗装置的步进源。在矩阵移位方法中，假设辐射源是固定的，而是根据步数移动数据矩阵。在本研究中，通过针对一个点运行MCNPX程序并使用矩阵移位方法计算剂量矩阵，获得了4厘米和6厘米食管癌肿瘤长度的等剂量曲线，并与分别在每个步位运行MCNPX程序获得的等剂量曲线进行比较（4厘米和6厘米食管癌肿瘤长度分别为15步和23步）。
基于平均剂量差异，步进法和矩阵移位法的两个剂量矩阵之间的差异，对于4厘米和6厘米的治疗长度分别为3.85×10⁻⁴戈瑞和5.19×10⁻⁴戈瑞。剂量差异不显著，这两种方法同样有效。
本研究中提出的矩阵移位方法可用于计算近距离放射治疗步进源的剂量分布，作为一种比其他基于蒙特卡罗的常规方法更快的工具。
