Animal-Type Melanoma: A Rare Type of Malignant Melanoma with an Indolent Clinical Behaviour

Animal-type melanoma is an exceedingly rare histological variant of melanoma in humans. The name was coined to reflect similar histological features to melanomas in grey horses. We present a case of animal-type melanoma. The neoplastic cells were heavily pigmented with an epithelioid morphology, round nuclei and prominent eosinophilic nucleoli. Only occasional mitotic figures were identified. The tumour cells had diffuse and nodular growth patterns with involvement of the dermis and the subcutaneous tissue. The patient had a "benign cellular blue naevus" excised 9 years earlier from the same site. Review of the previous case revealed heavily pigmented epithelioid cells with similar morphology to the current case. Nuclear pleomorphism was minimal and only a single mitotic figure was present. The lesion extended to the margins of excision. The diagnosis of animal-type melanoma was made on the current case and on the previously misdiagnosed case. Recurrence of this case nine years following incomplete excision further supports the hypothesis that animal-type melanoma is a distinct histological type of malignant melanoma with an indolent clinical course.