Single dose irradiation response of pig skin: a comparison of brachytherapy using a single, high dose rate iridium-192 stepping source with 200 kV X-rays

An experimental brachytherapy model has been developed to study acute and l ate normal tissue reactions as a tool to examine the effects of clinically relevant multifractionation schedules. Pig skin was used as a model since i ts morphology, structure, cell kinetics and radiation-induced responses are similar to human skin. Brachytherapy was performed using a microSelectron high dose rate (HDR) afterloading machine with a single stepping source and a custom-made template. In this study the acute epidermal reactions of ery thema and moist desquamation and the late dermal reactions of dusky mauve e rythema and necrosis were evaluated after single doses of irradiation over a follow-up period of 16 weeks. The major aims of this work were: (a) to co mpare the effects of iridium-192 (Ir-192) irradiation with effects after X- irradiation; (b) to compare the skin reactions in Yorkshire and Large White gigs; and (c) to standardize the methodology. For Ir-192 irradiation with 100% isodose at the skin surface, the 95% isodose was estimated at the basa l membrane. while the 80% isodose covered the dermal fat layers. After HDR Ir-192 irradiation of Yorkshire pig skin the ED50 values (95% isodose) for moderate/severe erythema and moist desquamation were 24.8 Gy and 31.9 Gy, r espectively. The associated mean latent period (+/- SD) was 39 +/- 7 days f or both skin reactions. Late skin responses of dusky mauve erythema and der mal necrosis were characterized by ED50 values (80% isodose) of 16.3 Gy and 19.5 Gy, with latent periods of 58 +/- 7 days and 76 +/- 12 days, respecti vely. After X-irradiation, the incidence of the various skin reactions and their latent periods were similar. Acute and late reactions were well separ ated in time. The occurrence of skin reactions and the incidence of effects were comparable in Yorkshire and Large White pigs for both X-irradiation a nd HDR Ir-192 brachytherapy. This pig skin model is feasible for future stu dies on clinically relevant multifractionation schedules in a brachytherapy setting.