Assessment of the absorbed dose uncertainty of prostate due to tissue swelling and ‎radioactive sources displacement in brachytherapy using Monte Carlo method

Brachytherapy is a kind of cancer treatment in  which radiation sources are implanted inside or close to the cancerous tissue. ‎The purpose of this research is to calculate the absorbed dose uncertainty of prostate tissue, due to its swelling, ‎displacement of the implanted seed sources and also, to address the effect of these factors simultaneously, in brachytherapy of prostate. ‎In this research, MCNPX2.6 code, the TG-43U1 protocol and ORNL body phantom were used to simulate the brachytherapy ‎of prostate using iodine-125 seed sources. In the first study, 84 sources of iodine with the  shapes of seed and then points  were ‎implanted inside the prostate with the volume of 38.01 cm3. The radiation absorbed dose was found to be 110.59 and ‎‎110.57 Gy, respectively. Considering the 50% prostate inflation after implantation, the radiation absorbed doses of prostate ‎showed a reduction of  more than of 17%. In the second therapeutic plan, by using 76 seed sources of I-125, considering 12% ‎swelling of prostate and applying the  displacement of seed sources in three directions: left–right (1.8mm), front-back (2.1mm) and top–‎down (3.4mm)),  the radiation dose amount of the cancerous tissue was reduced about 21%. So, the results ‎of seed and point sources of brachytherapy were very close to each other. Therefore, in simulation studies, point sources can be used ‎instead of seed sources to  reduce the computational complexity. Also,  this research showed the effects of swelling and ‎displacement of brachytherapy sources on  the amount of  the absorbed dose of prostate and its treatment were‎ noteworthy.