Abstract

Cytokeratin-20 (CK20) immunohistochemistry (IHC) is useful in the diagnosis of Merkel cell carcinoma (MCC), with a characteristic perinuclear dot-like pattern (PNDLP) of expression. Prior studies and our experience suggest that other CK20 IHC patterns, including membranous, cytoplasmic, and mixed, may also be seen in MCCs. MCCs with non-PNDLP CK20 IHC have been reported to occur less commonly on sun-exposed skin of the head and neck, but it is unknown whether a lack of PNDLP CK20 IHC is associated with Merkel cell polyomavirus (MCPyV) status. We conducted a retrospective cohort analysis of MCC cases for which both CK20 and MCPyV IHC were performed at our institution. We hypothesized that MCCs with non-PNDLP CK20 IHC would be more likely found on non-sun exposed skin and more likely positive for MCPyV. Of 16 cases, 9 exhibited predominantly PNDLP CK20 IHC, while 6 exhibited a mixed pattern, and 1 was negative for CK20 IHC. We found that CK20 IHC pattern was not associated with MCPyV status (p = 0.28) or location on the head and neck (p = 0.61). In conclusion, CK20 IHC pattern does not seem to correlate with MCC etiology in our study, but it is nevertheless important for pathologists to be aware of the multiple staining patterns observed in MCC.