Can Immunohistochemistry Replace Molecular Test in Diagnosing Mammary Analog Secretory Carcinoma?

Mammary analog secretory carcinoma (MASC) is a distinct variant of rare, low-grade salivary gland carcinoma with characteristic genetic alteration and the ETV6-NTRK3 gene fusion. MASC is recently described in the new WHO classification of head-and-neck tumors (4th edition, 2017). The tumor has the similar morphologic, immunohistochemistry (IHC), and molecular features of mammary secretory carcinoma. Immunohistochemically, the tumor cells display strong positivity for CK7, mammaglobin, S100, MUC-4, and STAT5. In resource-constraint centers, IHC plays an important role in diagnosing MASC; however, cases with discrepancies between morphological and immunohistochemical expression, confirmation of ETV6-NTRK3 fusion gene is a must. Herein, we describe this rare entity in a young female with parotid region swelling, which on gross examination was encapsulated and on microscopy had a predominant microcystic pattern comprising polygonal cells, which were immunopositive for S100, mammaglobin, and CK7 while negative for DOG 1. Based on these findings, a final diagnosis of MASC was rendered.