Merkel cell carcinoma of the eyelid: histological and immunohistochemical features with special respect to differential diagnosis

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Abstract
· Background: Merkel cell carcinomas (MCC) not infrequently involve the periorbital region and the eyelids. Clinically, they are relatively characteristic but often unsuspected. Histologically, MCC are often misdiagnosed as lymphoma, melanoma, or metastatic small cell carcinoma of the lung (SCCL). · Methods: We present clinical, histological, and immunohistochemical data on six eyelid cases (all females; age 63–102 years; one with concomitant CLL) from our files of 77 MCC with special respect to differential diagnosis. For comparison, 22 SCCL were analyzed. Immunohistochemistry was done with antibodies against pan-cytokeratin (pan-CK), cytokeratin-20 (CK-20), neurofilament protein (NF), neuron-specific enolase (NSE), chromogranin (CHR), and S100 protein (S100). · Results: Morphologically, five of six MCC were prototypic, one was of the small cell variant. Immunohistochemically, dot-like positivities for pan-CK and CK-20 were seen in all six MCC, and for NF in five tumors. None of the 22 SCCL stained positively for CK-20 or NF but 21/22 cases were positive for pan-CK. Only 1/21 SCCL showed dot-like patterns for pan-CK; 20/21 reacted diffusely. All MCC and 13/22 SCCL displayed CHR-positive cells. All MCC and all SCCL were positive for NSE and negative for S100. · Conclusions: Dot-like positivities for CK-20 or NF are important to prove MCC and to exclude SCCL in clinically and morphologically doubtful cases. Dot-like positivities for pan-CK favor MCC, but do not always exclude SCCL. NSE and CHR are of no value for the differential diagnosis of MCC and SCCL. Melanoma and lymphoma are ruled out by negativity for S100 and pan-CK, respectively.

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