Malignant melanoma with prominent pigment synthesis: "animal type" melanoma--a clinical and histological study of six cases with a consideration of other melanocytic neoplasms with prominent pigment synthesis.

Rare skin neoplasms in humans, comprising nodules of heavily melanized cells, mimic melanocytic neoplasms seen in horses and laboratory animals and thus are termed animal type melanomas. In part because of their rarity, behavior is unpredictable; many cases manifest a long indolent phase, and metastases are reportable. Over 6 years, the authors encountered nine skin and one lymph node biopsy specimens from six patients in whom light microscopy of formalin-fixed, paraffin-embedded tissue sections stained with hematoxylin and eosin showed melanocytic neoplasms with prominent pigment synthesis. Clinical follow-up was obtained by telephone contact with clinicians. There were three women, two men, and one boy, aged 9 to 85 years, whose lesions were described as blue-black nodules with irregular borders from 1.0 to 4.0 cm in size, located on the scalp, lower extremities, back, and sacrum. The dermatopathology comprised confluent dermal sheets of heavily melanized cells whose nuclei, where discernible, were large with irregularly thickened membranes, coarse chromatin, prominent, often spiculated nucleoli, and irregular parachromatinic clearing. Mitoses were infrequent. Four lesions had an epidermal component. One patient suffered metastases to regional lymph nodes, liver, and lungs with lethal effect, one experienced regional lymph node metastases but is still alive, one had local cutaneous metastases but was lost to follow-up, and one has a chest wall mass that has not yet been investigated. This rare dermal-based melanocytic neoplasm with prominent pigment synthesis, the animal type melanoma, has a biological behavior difficult to predict on morphological grounds. We advise complete excision with a 1.0- to 2.0-cm margin of normal skin and clinical investigation for regional or distant metastases.