The investigation of the effect of changes in seed location and orientation on dose distributions in low dose-rate brachytherapy treatment using monte carlo method

The aim of this study is to investigate the effects of random translations and rotations of multiple brachytherapy sources inside the prostate volume on the dose distributions within the patent's body. That is usually a result of organ motions which is encountered in rutin brachytherapy treatment where low-dose-rate(LDR) radioactive seed sources are used. Dosimetric calculations in this study, carried out by Monte Carlo particle transport technique, were performed with BrachyDose code which is a model-based dose calculation algorithm. In the radiaton dose calculations, based on the recommendations in both the TG-43 and TG-186 reports of AAPM, two different virtual phantom models each is consist of 1.0×1.0×1.0 mm3 voxel dimensions were used. A total of six different LDR commercial brachytherapy source models with different internal component geometries were used. Three of these source models contain 125I radioisotope and the other three source models contain 103Pd radioaisotope as the radiation source. Dosimetric analyses were performed on dose-volume histograms (DVH) and two-dimensional voxel-based dose change graphs comparatively. When the results obtained from this study were evaluated, it was observed for the prostate gland that there were dose differences ranging from 1.1% and 3.5% (maximum 5.5%) on average between the planned treatment dose and the dose delivered to the patient, due to the random displacement and rotation of the brachytherapy sources placed inside the prostate. Dose differences obtained for some risky organs around the prostate, such as the bladder wall, rectum, vesicle, pubic bone and urethra, were averagely calculated as 2.9%, 2.7%, 6.8%, 4.5% and 12.5%, respectively. It was observed that there were significant dose differences in the dose distributions within the environment around the seed sources due to the random motions of brachytherapy sources in the prostate volume. We concluded that the results obtained in this study are suitable to be considered as a data source for the success of prostate brachytherapy applications in rutin treatment.