Merkel cell carcinoma with eccrine differentiation: a clinicopathologic study of 7 cases.

We described 7 examples of Merkel cell carcinoma of the skin with eccrine and squamous differentiation. Five patients were men, and 2 were women; and their ages ranged from 63 to 81 years (mean age, 73 years). Six tumors arose in the head and neck; and one, on the sole of the right foot. Three tumors recurred locally, and 2 metastasized to the regional lymph nodes. No patient developed distant metastasis. Two patients died of unrelated causes. Five Merkel cell carcinomas showed classic cytology, and 2 were similar to small cell carcinomas of the lung. All 7 tumors showed small eccrine ducts, and 2 exhibited foci of squamous differentiation. The eccrine ducts label with cytokeratin 7 and carcinoembryonic antigen, whereas the predominant endocrine component displayed the characteristic paranuclear dot-like reactivity with cytokeratin 20 and was synaptophysin and chromogranin positive. The lymph node metastasis contained both eccrine ducts and squamous elements, suggesting that they are an integral component of the tumors. Eccrine differentiation in Merkel cell carcinomas similar to small cell carcinomas of the lung and extrapulmonary sites is an important feature in the differential diagnosis because eccrine differentiation has not been described in primary or metastatic small cell carcinomas. The prognosis of these Merkel cell carcinomas with divergent differentiation appears to be less aggressive than that of pure Merkel cell carcinomas. However, larger series of patients with longer follow-ups are needed to confirm this observation.