Signet cell melanocytic lesions.

Malignant melanoma can produce diagnostic problems for the histopathologist because of its protean histologic patterns. The recently recognized signet cell pattern can be particularly confusing and must be distinguished from adenocarcinoma, tumors of vascular endothelium or adipose tissue, lymphoma, and epithelioid smooth muscle lesions. We report four new cases of signet cell melanoma and illustrate this pattern in primary as well as metastatic sites. In addition, we document the signet cell pattern in benign nevi for the first time, expanding the concept of this pattern to melanocytic cells in general. The differential diagnosis of signet cell melanoma and its mimics is discussed and the utility of immunohistochemical stains in this diagnosis is stressed.