Merkel cell carcinoma with squamous differentiation - case report.

We report a case of 81-year old female with Merkel cell carcinoma of skin with squamous differentiation and aberrant immunohistochemical profile. Primary skin tumor of left preauricular area was incompletely removed 6 months ago and diagnosed as poorly differentiated squamous cell carcinoma. After 6 months patient returns with relapse and new biopsy is made. Microscopically, mostly solid structures of poorly differentiated neoplasia with focal squamous differentiated are found. Neoplastic cells were small, polygonal, with scant cytoplasm and irregular oval dark nuclei with high mitotic activity. Immunohistochemically, tumor cells showed positivity for AE1/AE3, p63, CD56, chromogranin A, synaptophysin and in squamous parts p40. CK20 and other markers examined were negative. Merkel cell carcinoma is rare and highly aggressive skin tumor with characteristic expression of epithelial and neuroendocrine markers. In this report we want to point out that caution should be taken in excluding diagnosis of Merkel cell carcinoma from differential diagnosis based on atypical histological and immunohistochemical findings, especially the risk of confusion with less aggressive skin tumors.