MALIGNANT-MELANOMA WITH CLINICAL AND HISTOLOGIC FEATURES OF MERKEL CELL-CARCINOMA

We describe a patient with malignant melanoma that resembled a Merkel cell carcinoma both clinically and histologically. Immunohistochemical studies showed focally positive staining with S-100 protein and stron gly positive staining with HMB-45. Ultrastructural study confirmed the diagnosis by demonstrating premelanosomes and melanosomes. Although t he tumor appeared to be clinically unimpressive, it was a deep melanom a with a Breslow level of 3.8 mm that necessitated aggressive treatmen t. Small cell melanoma must be considered in the differential diagnosi s of small cell tumors, which also includes lymphoma, eccrine carcinom a, squamous cell carcinoma, and Merkel cell carcinoma. The diagnosis o f amelanotic melanoma, including the small cell variant, may require e lectron microscopic studies.