Animal type melanoma: an unusual case with aggressive histological features?

Animal-type melanoma (ATM) refers to a well-known but rare, heavily pigmented melanocytic tumor, considered a variant of malignant melanoma, the biological behavior and prognostic significance of which still remain to be completely established. We report a case characterized by proliferation of hyperpigmented epithelioid and spindle cells, focally ulcerating the epidermis. Tumor necrosis, perineural and vascular invasion, as well as a slightly high mitotic index, were also observed. Although there were several features indicating poor prognosis, the lesion was diagnosed as ATM and not as classical melanoma. This almost unique case could help us to confirm the excellent biological behavior of this "uncertain malignant potential tumor", suggesting a biological nature of ATM possibly different from classical melanoma, as reported in other molecular studies. In addition, the first rapid impression of aggressive melanoma could lead the pathologist to render an immediate, incorrect diagnosis.