Cutaneous malignant melanomas from 81 rabbits were retrospectively evaluated, and 51 cases were re-examined to elucidate their histopathological and immunohistochemical characteristics and to identify potential associated prognostic factors. The mean age of rabbits at tumor incidence was 6 years, 9 months (median age: 7 years; range: 2 years, 1 month to 12 years, 4 months). Netherland dwarfs and intact males were more prevalent in terms of breed and sex, respectively. The most common tumor location was the scrotum, followed by the head, including the eyelids and pinna, and trunk. The tumors were composed of 3 cell types: epithelioid, spindle, and mixed. Histopathological parameters examined included mitotic counts, nuclear atypia, multinucleated giant cells, degree of pigmentation, local invasion, tissue margins, presence of satellite nodules, vascular invasion, intralesional necrosis, and intralesional inflammation. In this study, most histopathological parameters were not associated with a shorter survival time. Maximum tumor diameter and mitotic counts were associated with a poor prognosis, with cutoffs of 2.0 cm and 10 mitoses per 2.37 mm 2 , respectively. Immunohistochemically, 51 of 51 cases (100%), 50 of 51 cases (98%), and 49 of 51 cases (96%) were positive for PNL2, melan-A, and HMB-45, respectively. No apparent differences in positivity were observed among the cell types of the neoplasms. This study provides several new insights into malignant melanomas of rabbits, such as breed, anatomical site, and sex predilections, and detailed histopathology, including useful cutoff values of associated prognostic factors.
Kondo et al. (Fri,) studied this question.