Case report of Mammary Analog Secretory Carcinoma of the parotid gland.

Mammary Analog Secretory Carcinoma (MASC) is a new entity of malignant salivary gland tumors that morphologically resembles mammary secretory carcinoma and carries the identical ETV6-NTRK3 fusion gene. We report our first case of MASC in Japan occurring in the parotid gland of a 37-year-old female patient with a t (12; 15) (p13; q25) translocation. Histologically, the tumor was composed of monomorphic cuboidal cells with low-grade vesicular nuclei and pale eosinophilic cytoplasm, and formed microcystic and tubular spaces with periodic acid-Schiff-positive secretion. Immunohistochemically, the tumor cells tested positive for cytokeratin, vimentin, and S-100 protein. MASC is a morphological mimicker of acinic cell carcinoma, but is a distinct neoplasm characterized by a specific chromosomal translocation. An accumulation of similar case studies is mandatory in order to clarify biological behaviors.