Mammary analog secretory carcinoma (MASC) is a recently described tumor predominantly arising in the parotid gland. These tumors represent locally invasive malignancies with microcystic architecture, low-grade nuclei, and granular pink vacuolated cytoplasm. They display strong vimentin and S100 positivity and harbor an identical t(12;15)(p13;q25) to their breast counterpart, leading to a ETV6-NTRK3 fusion oncogene. These features help exclude the most important differential diagnostic considerations, namely, acinic cell carcinoma (AciCC) and low-grade cystadenocarcinoma, not otherwise specified. Here we present a series of 7 recent examples of MASC, which showed features not previously described. These 7 cases were observed in patients ranging in age from 14 to 77 years (mean, 40 y), occurred almost exclusively in male patients (6:1), and showed >50% (4 of 7 cases) involvement of the oral cavity, with only 2 arising in the parotid. The remaining case is the first reported in the submandibular gland. The tumors showed a variety of patterns including single macrocysts, combined macrocystic and microcystic spaces, and solid architecture. They showed prominent hobnailing in the cystic areas. Secretions within the cysts and tubular areas tended to be positive for periodic acid schiff, periodic acid schiff diastage and mucicarmine, the latter also showing occasional intracytoplasmic mucin droplets, a feature not previously recognized. One case showed prominent mucinous differentiation, which, coupled with high-molecular-weight keratins (HMWK) positivity, mimicked mucoepidermoid carcinoma (MEC). The tumors were generally positive for HMWK (6 of 7), S100 (5 of 7), vimentin, CK19, and other epithelial markers. The finding of duct involvement, proven with an incomplete p63-positive basal layer surrounding a minority of tumor cell nests and cysts, raised the possibility of a ductal epithelial origin for MASC. Alternatively, this could represent secondary ductal involvement by tumor. All cases showed rearrangement of the ETV6 gene by fluorescence in situ hybridization, confirming the diagnosis of MASC. These findings reinforce MASC as a unique low-grade salivary gland tumor entity with morphologic overlap with AciCC, MEC, and cystadenocarcinoma.
*Department of Pathology, University Health Network
†Department of Laboratory Medicine and Pathobiology, University of Toronto
‡Cytogenetics Laboratory, Department of Paediatric Laboratory Medicine, The Hospital for Sick Children, Toronto, ON, Canada
§Department of Pathology, Medical Faculty of Charles University, Plzen, Czech Republic
Conflicts of Interest and Source of Funding: The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this study.
Correspondence: Ilan Weinreb, MD, FRC(P)C, Department of Pathology, University Health Network, 200 Elizabeth Street, Toronto, ON, Canada, M5G 2C4 (e-mail: email@example.com).