Malignant melanoma with clinical and histologic features of Merkel cell carcinoma

Nancy S. House, Pennsylvania State University
Fred G. Fedok, Pennsylvania State University
Mary E. Maloney, University of Massachusetts Medical School
Klaus F. Helm, Pennsylvania State University

At the time of publication, Mary E. Maloney was not yet affiliated with the University of Massachusetts Medical School.

Abstract

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 strongly positive staining with HMB-45. Ultrastructural study confirmed the diagnosis by demonstrating premelanosomes and melanosomes. Although the tumor appeared to be clinically unimpressive, it was a deep melanoma with a Breslow level of 3.8 mm that necessitated aggressive treatment. Small cell melanoma must be considered in the differential diagnosis of small cell tumors, which also includes lymphoma, eccrine carcinoma, squamous cell carcinoma, and Merkel cell carcinoma. The diagnosis of amelanotic melanoma, including the small cell variant, may require electron microscopic studies.