Syringomatous Tumor Originating From an Axillary Supernumerary Mammary Tissue: First Report of an Axillary Tumor Case

Syringomatous tumor typically occurs in the nipple of female patients and is regarded as a breast-specific tumor rather than a cutaneous adnexal tumor, owing to its nipple location and its differentiation toward large breast ducts. Herein, we report the first axillary case of syringomatous tumor arising from supernumerary mammary tissue lacking a supernumerary nipple. A 25-year-old woman presented with a 7-mm nodule in the left axilla. Histopathologic examination revealed a poorly circumscribed tumor composed of small tubules embedded in abundant fibrosing stroma, extending from the mid-dermis to superficial subcutis. The tumor tubules demonstrated showed a 2-cell layer consisting of cuboidal glandular epithelial cells and flattened myoepithelial cells with minimal atypia. Keratinous cysts were scattered throughout the superficial portion of the tumor. Medium-sized ductal structures suggestive of mammary ducts were identified within and at the periphery of the tumor. Diffuse immunoexpression of cytokeratin 5/6, cytokeratin 7, and cytokeratin 19 was observed. Most tumor cells also expressed S100 protein, SOX10, and c-kit, whereas estrogen receptor positivity was limited in the tumor component. α-Smooth muscle actin and p63 highlighted the peripheral myoepithelial cell layer of the tumor tubules. This case demonstrates that syringomatous tumor can arise in supernumerary breast tissue lacking a nipple. Histopathologically, syringomatous tumor may resemble cutaneous adnexal tumors, particularly syringoma and microcystic adnexal carcinoma, and breast carcinomas. However, in addition to the clinical appearance and location, the presence of a peripheral myoepithelial lining and immunoexpression of SOX10 and c-kit may help exclude these entities.