Assessment of absorbed dose uncertainty of prostate due to source relocating in brachytherapy by monte carlo method

Brachytherapy, is a method to treat prostate cancer in which a radiation source is placed inside or next to the cancer affected tissues. Purpose of this article is to determine the uncertainty level of received dose in prostate tissue due to the relocation of the placed radiation sources and inflation in prostate after positioning the radiation seeds. To simulate the model 6711 Amersham (activity level equal to 0.5 mCi) iodine-125 brachytherapy source, we used MCNPX 2.6 code and TG-43U1 protocol, and we used ORNL phantom. Treatment plan included 76 sources consisting iodine-125 radioactive, once placed in the form of seeds and once applied in the form of beads in three initial volume: 30.02, 38.01 and 52.01 cm3 and three different time steps were placed in prostate phantom. Three-time steps were investigated, including the moment after placing, the zero days and thirty days after placing. In the first step, the obtained dose of radiation in three different prostate volumes, for both seed distribution and bead distribution in healthy and cancerous tissue, was calculated. In the second step, position of the sources was relocated from the assumption, and causes a change in dose distribution. Considering the movements in 3 directions of left-to-right (1.8 mm), intra-exterior (2.1 mm), up-down (3.4 mm) after implantation the equivalent dose was calculated. The maximum uncertainty of received dose in a prostate with seed implanted sources and the volume of 52.01 cm3 and it was reported as +20%. At the third step, in addition to the change in the location of sources, inflation rate of 12% was observed in comparison with the primary volume at the first step of treatment process which resulted in a smaller dose rather than the previous stage. In this treatment plan, relocating of the sources and 12% inflation of the tumor were analyzed. Uncertainty of source relocating was calculated as ±20% in zero-day of positioning. Also, 12% inflation of the tumor and source relocation after 30 days, in a prostate with the volume of 58.25 cm3 curing by point sources, shown 21.04% reduction in received dose for healthy and cancer tissues since the moment of placing. The maximum error of MCNPX code was calculated 0.03%.