Mucoepidermoid carcinoma of the skin: a distinct entity from adenosquamous carcinoma: a case study with a review of the literature.

Mucoepidermoid carcinoma (MEC) of the skin is an exceedingly rare but distinctive neoplasm with respect to its histopathologic features. It is similar if not identical in most respects to MEC of the salivary gland, a neoplasm whose prognosis is correlated with the pathologic grade. We report a case of MEC of the skin in a 79-year-old white woman who presented with an axillary mass. Beneath an unremarkable epidermis, a circumscribed, cystic neoplasm, unattached to the surface, was characterized by the presence of vague lobules of low-grade-appearing squamous cells accompanied by mucigenic and clear cells. A mucin stain highlighted the mucigenic cells and immunohistochemistry revealed pan-cytokeratin, cytokeratin 7, polyclonal carcinoembryonic antigen, and epithelial membrane antigen positivity. The cytokeratin 20 and gross cystic disease fluid protein were nonreactive. Inconsistency was encountered in the literature where some confusion existed as to whether MEC is synonymous with adenosquamous carcinoma of the skin. Elsewhere in the body, the latter tumor type is a squamous and gland-forming neoplasm with intermediate- to high-grade features rather than a tumor with mucigenic cells intermingled among intermediate and squamous cells. As in the case of MEC and adenosquamous carcinoma elsewhere in extracutaneous sites, we would propose that a pathologic distinction should be made in the skin for the sake of consistency and for prognostic purposes. Additionally, the immunophenotype of our case is similar to at least two other cases of cutaneous MEC, as well as MEC of the salivary gland, to support the hypothesis that this neoplasm is adnexal rather than epidermal in origin.