title: Monte Carlo dose calculation of GZP6 (60)Co stepping source based on a matrix shift technique.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  As an alternative method, a matrix shift based technique was applied to simulate a stepping source for brachytherapy.
  
  
  METHODS
  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).
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Medical Physics Research Center, Medical Physics Department, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran.
authors.forename: Mohammad Taghi Bahreyni
authors.initials: MT
authors.lastname: Toossi
authors.affiliation: Medical Physics Research Center, Medical Physics Department, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran.
authors.forename: Malihe
authors.initials: M
authors.lastname: Abdollahi
authors.affiliation: Medical Physics Research Center, Medical Physics Department, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran.
authors.forename: Mahdi
authors.initials: M
authors.lastname: Ghorbani
keywords: Brachytherapy; Co-60; Matrix shift based technique; Monte Carlo simulation; Stepping source