An FNA pitfall: Mammary analog secretory carcinoma mistaken for acinic cell carcinoma due to cytoplasmic granules

In the salivary gland, a key differential feature of Mammary analog secretory carcinoma (MASC) from acinic cell carcinoma (ACC) is the lack of cytoplasmic granules. We report a case of a parotid mass incorrectly diagnosed on fine needle aspirate as acinic cell carcinoma due to many cells with basophilic granules suggesting serous acinar differention. Tumor resection revealed a tumor consistent with low grade adenocarcinoma that had eosinophilic, microvacuolar cytoplasm with distinct basophilic granules staining with PASD and mucicarmine. The diagnosis of MASC was confirmed with stains for GCDF-15, mammoglobin, and S100 and FISH consistent with a t(12;15) translocation. Relying on the absence of cytoplasmic granules as a feature to distinguish ACC from MASC is a diagnostic pitfall.