A Cutaneous Myoepithelial Carcinoma Arising in a Papillary Eccrine Adenoma

Benign and malignant neoplasm of myoepithelial cells comprise a rare but well-characterized group of tumors in the salivary glands, breasts, and soft tissues. Histopathologically, myoepitheliomas show solid, reticular, trabecular architecture and myxoid or hyaline stroma, and are composed of round/epithelioid or spindle cells. The tumor cells of myoepitheliomas possess the capacity for epithelial and myoid differentiation and, as a consequence, immunohistochemical reactivity is variable, showing combinations of immunoexpression for cytokeratin, epithelial membrane antigen (EMA), S-100 protein, α-smooth muscle actin (α-SMA), calponin, and glial fibrillary acidic protein (GFAP). In normal skin, myoepithelial cells are present as spindle-shaped cells arranged as a discontinuous peripheral layer around eccrine and apocrine glands; their contractions aid delivery of the secretory products of these glands. Only recently have neoplasms been shown to compose only myoepithelial cells that occur primarily in the skin. Because fewer than 50 such cases have been published to date, limited data are available regarding the morphological spectrum and immunophenotype of these cells. Moreover, the criteria for differentiating benign form from malignant myoepithelioma have not been fully clarified. The papillary eccrine adenoma (PEA) was first described by Rulon and Helwig in 1977; it is a rare, benign, slow-growing tumor. Clinically, the tumor presents as a solitary dermal nodule and is most commonly located on the extremities. Histopathologically, the tumor is composed of multiple, dilated tubular structures lined by two or more layers of epithelial cells. However, malignant change of a PEA is extremely rare, with only one case reported to date. Here, we report an extremely rare case of cutaneous myoepithelial carcinoma arising in a PEA with lymph node metastasis. To the best of our knowledge, this is the fifth case of a myoepithelial carcinoma and the second case of malignant change of a PEA reported in the medical literature.