Abstract

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine tumor. Risk factors for tumorigenesis include advanced age, ultraviolet (UV) exposure, immunosuppression, and infection with Merkel cell polyomavirus (MCPyV). Occasionally, morphologic diversity or divergent differentiation may be noted, particularly among MCPyV-negative cases; of which squamous differentiation is the most common. We present two cases of MCPyV-negative MCC with eccrine/ductal differentiation, which is extremely rare and can potentially pose diagnostic challenges. Case 1: An 81-year-old man presented with a 1.5 cm pink papule on his lower lip. A shave biopsy revealed an infiltrative high-grade basaloid tumor with morphologic and immunophenotypic features diagnostic of MCC. In addition, the lesion exhibited focal clusters of CK7+ squamous cells with central CEA+ ductal structures. This patient succumbed to progressive MCC, one year after diagnosis. Case 2: A 67-year-old man presented with a violaceous papule on his right forearm. A shave biopsy demonstrated an infiltrative basaloid tumor with morphologic and immunophenotypic features diagnostic of MCC. In addition, the lesion demonstrated areas of squamous differentiation and scattered CEA+ ductal structures. He is being treated with carboplatin, etoposide and pembrolizumab therapy for progressive MCC. Although eccrine/ ductal differentiation is unusual, the other histologic and immunophenotypic features support the diagnosis of MCC in both cases. Divergent differentiation is rare among MCC, and can pose diagnostic challenges, especially when prominent. Awareness of this phenomenon is essential for accurate diagnosis and optimal patient management. Studies with large patient cohorts are necessary to determine the prognostic importance of the various histologic patterns of divergent differentiation in MCC.

Financial Disclosure:
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