Dose response for ultraviolet radiation A-induced focal melanocytic hyperplasia and nonmelanoma skin tumors in Monodelphis domestica

Four groups of 30 dorsally shaved opossums (Monodelphis domestica) were exp osed to graded doses of ultraviolet radiation A (UVA) (320-400 nm) three ti mes per week for 90 weeks. Animals were monitored for the appearance of foc al melanocytic hyperplasia (FMH) and nonmelanoma skin tumors (NMST) during the course of the exposures and for an additional 20 weeks following termin ation of exposures, FMH is the putative precursor for melanoma in the oposs um, The lowest dose of UVA (2.5 x 10(3) J/m(2)) used in this study was sele cted based on the action spectrum for the induction of melanoma in a fish m odel, The prediction was that 2.5 x 10(3) J/m(2) would induce FMH in the op ossum if the action spectra for the induction of FMH in the opossum and mel anoma in the fish were the same. The highest UVA dose was 2.5 x 10(2) J/m(2 ). Only the highest dose of UVA gave a statistically significant induction of FMH and NMST in the opossum, As in previous studies, the FMH appeared ea rlier than the NMST during the course of exposures and the final prevalence of FMH was lower than the final prevalence of NMST, Overall, the results o f this study indicate that the efficacy of UVA to induce FMH in the opossum is not as great as would be predicted from the action spectrum for melanom a induction in a fish model.