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 cell s, mimic melanocytic neoplasms seen in horses and laboratory animals and th us are termed animal type melanomas. In part because of their rarity, behav ior is unpredictable; many cases manifest a long indolent phase, and metast ases are reportable. Over 6 years, the authors encountered nine skin and on e 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. C linical 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 w ere 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 com prised confluent dermal sheets of heavily melanized ce lls whose nuclei, where discernible, were large with irregularly thickened membranes, coarse chromatin, prominent, often spiculated nucleoli, and irre gular parachromatinic clearing, Mitoses were infrequent. Four lesions had a n epidermal component. One patient suffered metastases to regional lymph no des, liver, and lung with lethal effect, one experienced regional lymph nod e 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 inv estigated. This rare dermal-based melanocytic neoplasm with prominent pigme nt 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. HUM PATHOL 30:543-550. Copyright (C) 1999 by W.B. Sa unders Company.