A case series and immunophenotypic analysis of CK20-/CK7+primary neuroendocrine carcinoma of the skin

Background: The diagnosis of Merkel cell carcinoma (MCC) can be rather challenging; therefore, the immunohistochemical profile is important in confirming the microscopic diagnosis. Characteristic of the neuroendocrine and epithelial differentiation of MCC, antibodies to cytokeratin (CK) 20, CK7, epithelial membrane antigen, and neuron-specific enolase among others, are used in confirming the diagnosis. As reported in the literature, the majority of MCC express CK20 and are CK7 negative. Herein, we present a case series of seven patients with CK20-/CK7+ primary cutaneous neuroendocrine carcinoma. Methods: All cases of MCC with specific CK20-/CK7+ staining on file at a large veterans' hospital and tertiary referral dermatopathology service were reviewed. All seven cases were analyzed for clinical, pathologic and immunophenotypic attributes. All specimens were submitted for immunohistochemical staining and interpreted by a single dermatopathologist. Results: All the cases showed diffuse cytoplasmic staining for CK7 and positive staining for synaptophysin. CK20 and thyroid transcription factor-1 staining was negative. Conclusions: Herein we have presented a hitherto unreported group of patients with CK20-/CK7+ primary neuroendocrine carcinoma of the skin. The purpose of this case series is to describe a new immunophenotypic variant of MCC, while further expanding the differential diagnosis of tumors included in the subset of neoplasms showing CK20-/CK7+ staining.