CK20 negative and CK7 positive merkel cell carcinoma of the conjunctiva: Brief case report

Merkel cell carcinoma (MCC) is an uncommon but potentially aggressive neuroendocrine carcinoma of the skin. It typically develops on sun-exposed areas of the head and neck, particularly the eyelid, periorbital region, and extremities. We report a case of unusually located MCC, presenting as a conjunctival mass, which has only been reported once in the English literature. An 83-year-old male presented with a 0.8 × 0.7 cm sized mass protruding from the lower fornix of the right conjunctiva. The mass was excised. The tumor was located in the sub-mucosa without connection to the overlying mucosa. Light microscopic findings showed a carcinoma with undifferentiated small round cells and the presence of cytokeratin (CK AE1/3, CK7) and neuroendocrine markers. We established a diagnosis of MCC. As reported in the literature, the majority of MCCs are positive for CK20 but negative for CK7. But, this case showed an uncommon cytokeratin immunohistochemical profile of positive for CK7 and negative for CK20, suggesting a new immunophenotypic MCC variant.