Monte Carlo based determination of dose distribution for some patch sources employed for radionuclide skin therapy

Several methods are introduced for skin cancer treatment. An encouraging method is radionuclide skin therapy, where high-energy beta emitting radionuclides such as ³²P, ⁹⁰Y, ¹⁸⁸Re and ¹⁶⁶Ho are employed for skin irradiation. This study aimed to calculate the dose distribution for mentioned radionuclides at different layers of skin phantom through Monte Carlo simulation. Depth dose distribution, transverse dose profile and isodose curves related to the patch sources under investigation were calculated by MCNPX code. All of calculations were performed inside a developed skin phantom. Obtained results were compared with those reported by other studies to evaluate the validity of simulations. The results showed that the ¹⁶⁶Ho and ³²P have steeper dose gradient within the depth which can lead to the better normal tissue sparing. Simulated depth dose distributions were in a good agreement with other published studies and confirmed the validity of performed simulations. The obtained transverse dose profiles at 0.2 mm depth had acceptable symmetry and flatness that can improve the dose uniformity within the target area. Calculated isodose curves showed that the 90% isodose level covers a circular area with the diameter of around 8 mm for all studied beta sources. From the results, it can be concluded that ¹⁶⁶Ho and ³²P are more effective in treatment of superficial skin lesions, while, ⁹⁰Y and ¹⁸⁸Re are more rational choices in treatment of deeply distributed skin tumors. Size of employed patch source should be based on the target area to minimize the delivered dose to the adjacent tissues.