Evaluation of bone dose arising from skin cancer brachytherapy: A comparison between Ir and Co sources through Monte Carlo simulations.

PURPOSE

This study aimed to calculate and compare absorbed dose to bone following exposure to Ir and Co sources in high dose rate (HDR) skin brachytherapy. Moreover, effects of the bone thickness and soft tissue thickness before the bone on absorbed dose to the bone are evaluated .

MATERIALS AND METHODS

Ir and Co sources inserted in Leipzig applicators with internal diameters of 1, 2 and 3 cm with/without their optimal flattening filters were simulated by MCNPX Monte Carlo code. Then, heterogeneous phantoms (including skin, soft tissue before and after the bone, cortical bone and bone marrow) were defined. Finally, relative depth dose values for the bone and other tissues in the heterogeneous phantoms were obtained and compared.

RESULTS

The average relative depth dose values of the skin, soft tissue before and after bone and bone marrow were almost similar for both Ir and Co sources, with a maximum difference less than 2%. However, a 0.1-6.8% difference was observed between average relative depth dose values of these two sources for the cortical bone. The results showed that with increasing the bone thickness and bone distance from the skin surface, the average relative depth dose values of the bone marrow and cortical bone decreased for both Ir and Co sources inserted in the applicators without/with their optimal flattening filters. For most of evaluated the applicators without/with their flattening filters, the average relative depth dose values of the bone marrow arisen from the Co source were higher than those obtained from the Ir source, while an opposite trend was observed for the cortical bone .

CONCLUSION

The obtained findings showed that the average relative depth dose values of Ir and Co sources at the corresponding depths of the designed heterogeneous phantoms were almost similar (expect for the cortical bone). Hence, it is concluded that Co source can be used instead of Ir source in HDR skin brachytherapy, particularly in developing countries.