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 (4^th 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.