Merkel cell carcinoma (primary neuroendocrine carcinoma of skin) mimicking basal cell carcinoma with review of different histopathologic features.

Merkel cell carcinoma (MCC) is a rare but highly aggressive malignancy, which often has typical histopathologic and immunohistochemical (IHC) features. Sometimes the diagnosis is missed because of atypical histological or aberrant IHC findings. A case of MCC that showed irregular lobules of basaloid cells with keratotic areas on the initial shave biopsy is being reported. IHC showed positive staining for high-molecular weight cytokeratin but negative staining for cytokeratin 20, findings consistent with basal cell carcinoma. Subsequent excision specimen showed histopathologic features more typical of MCC. IHC still was negative for cytokeratin 20 but positive for synaptophysin. Review of the literature shows other examples of MCC with basal cell carcinoma-like features. Various other histopathologic differentiations of MCC include those that demonstrate squamous cell and eccrine carcinoma features and those that show melanocytic, lymphomatous, sarcomatous, muscular, and atypical fibroxanthoma-like features. Different histopathologic patterns and mimics of MCC are reviewed to help prevent dermatopathologists from misdiagnosing this aggressive tumor.