Merkel cell carcinoma (MCC), an aggressive malignancy that has been increasing in incidence, rarely presents with an epidermotropic pattern.

We conducted an immunohistochemical evaluation of 6 previously unpublished cases of epidermotropic MCC, focusing particularly on the staining characteristics of epithelial membrane antigen and cytokeratin-20 in the hope of providing insight into the mechanism of epidermotropism in MCC.

This study is a retrospective evaluation using light microscopy and immunohistochemistry.

Forty cases of MCC with pathology at Rhode Island Hospital and the Miriam Hospital in Providence, RI, from 1983 through 2009 were reviewed. Following exclusion criteria, 6 patients (5 men, 1 woman) with a mean age of 82.5 years (range, 72-92) demonstrated epidermotropism. Three of 6 patients had MCC of the eyelid. In cases 1, 3, and 6, the perinuclear dot pattern observed with cytokeratin-20 in the epidermotropic MCC cells was less pronounced than the pattern observed in the dermis, and in all 6 of the tumors, the epidermal staining pattern observed with epithelial membrane antigen was not more or less prominent than the staining observed in the dermis.

The small total number of cases of epidermotropic MCC is a limitation.

The data presented reinforce the differential diagnosis of tumors with an epidermotropic growth pattern and the importance of immunohistochemical staining in the histologic workup of such tumors: squamous cell carcinoma in situ, melanoma, mycosis fungoides, eccrine porocarcinoma, sebaceous carcinoma of the eyelid, mammary and extramammary Paget disease, MCC, and epidermotropic metastases. It is notable that 3 of 6 identified tumors were located on the eyelid; further study of epidermotropic MCC may shed more light on this finding, either as an unusual coincidence or a finding with unexplained significance.