Animal-type melanoma: a clinical and histopathological study of 22 cases from a single institution

Summary Background Animal‐type melanoma is a rare distinct melanoma subtype, characterized by proliferation of heavily pigmented epithelioid and spindled melanocytes that resembles the heavily pigmented melanomas seen in grey horses. While animal‐type melanoma is generally considered to be more indolent than conventional melanoma, only a limited number of cases have been reported and, as such, the clinical characteristics of animal‐type melanoma are incompletely understood. Objectives To characterize the clinical and histopathological features of animal‐type melanoma, and determine any features that may predict outcome. Patients/Methods Data was extracted from a prospectively collected melanoma database (1994–2008), and a retrospective pathology database (1991–2008) for all patients with a diagnosis of both equivocal (8) and unequivocal (14) malignant animal‐type melanoma. We reviewed the clinical and histopathological features, including the sentinel lymph node biopsy (SLNB) status. Results A total of 22 patients were identified, with a median age of 35 years. The median Breslow depth was 2·22 mm. A SLNB was performed in 17 patients, eight (47%) were positive. Younger age was associated with: (i) animal‐type melanoma with features equivocal for malignancy (median age of 7 vs. 48 years, P = 0·01), and (ii) a negative SLNB (median age 12 vs. 53 years, P = 0·03). Four patients with unequivocal animal‐type melanoma developed recurrent metastatic disease, with one patient death. No patient with an equivocal animal‐type melanoma or negative SLNB developed recurrent disease; however, this did not reach statistical significance (P = 0·13 and P = 0·09, respectively). Conclusions Animal‐type melanoma has a propensity for regional lymphatic metastasis and is rarely capable of disseminated metastatic disease and death. Animal‐type melanoma appears to exhibit a spectrum of biological behaviour, with young patient age associated with more indolent disease.