title: A Monte Carlo study on dose distribution validation of GZP6 (60)Co stepping source.
abstract: OBJECTIVE
  Stepping source in brachytherapy systems is used to treat a target lesion longer than the effective treatment length of the source. Cancerous lesions in the cervix, esophagus and rectum are examples of such a target lesion.
  
  
  BACKGROUND
  In this study, the stepping source of a GZP6 afterloading intracavitary brachytherapy unit was simulated using Monte Carlo (MC) simulation and the results were used for the validation of the GZP6 treatment planning system (TPS).
  
  
  METHODS
  The stepping source was simulated using MCNPX Monte Carlo code. Dose distributions in the longitudinal plane were obtained by using a matrix shift method for esophageal tumor lengths of 8 and 10 cm. A mesh tally has been employed for the absorbed dose calculation in a cylindrical water phantom. A total of 5 × 10(8) photon histories were scored and the MC statistical error obtained was at the range of 0.008-3.5%, an average of 0.2%.
  
  
  RESULTS
  The acquired MC and TPS isodose curves were compared and it was shown that the dose distributions in the longitudinal plane were relatively coincidental. In the transverse direction, a maximum dose difference of 7% and 5% was observed for tumor lengths of 8 and 10 cm, respectively.
  
  
  CONCLUSIONS
  Considering that the certified source activity is given with ±10% uncertainty, the obtained difference is reasonable. It can be concluded that the accuracy of the dose distributions produced by GZP6 TPS for the stepping source is acceptable for its clinical applications.
authors.affiliation: Medical Physics Research Center, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran.
authors.forename: Mohammad Taghi
authors.initials: MT
authors.lastname: Bahreyni Toossi
authors.affiliation: Medical Physics Research Center, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran ; Medical Physics Department, Reza Radiation Oncology Center, Mashhad, Iran.
authors.forename: Maliheh
authors.initials: M
authors.lastname: Abdollahi
authors.affiliation: North Khorasan University of Medical Sciences, Bojnurd, Iran.
authors.forename: Mahdi
authors.initials: M
authors.lastname: Ghorbani
keywords: GZP6 brachytherapy system; High dose rate; Monte Carlo; Stepping source