Evaluation of radiation exposure in Ir-192 brachytherapy for treatment of keloids

Nippon Medical School Hospital recently developed a high dose rate superficial brachytherapy (HDR-SBT) for keloids, which has been carried out since 2008. In this treatment, Ir-192 sources are placed on the affected area through flexible tubes, which has the advantage of providing high dose concentration to complicated shapes. Obviously, unnecessary radiation exposure to high-energy gamma rays must be reduced for vital organs around the affected area. We have developed a system which can evaluate the radiation exposure of tissues and organs in a MIRD-5 phantom using the PHITS Monte Carlo code when the prescribed dose was delivered uniformly to a flat surface based on the actual geometry in HDR-SBT. In the PHITS simulation, an affected area was assumed on the chest of the MIRD-5 phantom, then the dwell time at each dwell position was optimized to deliver 6 Gy uniformly over the affected area in the actual geometry with the lead shields and the bolus. In addition, the radiation exposures of each tissue and organ in the MIRD-5 phantom during the irradiation were calculated. The biologically equivalent dose of the thymus was the highest at 1145.60 mGy followed by the heart at 236.14 mGy and the breasts at 95.32 mGy.