Cytopathology of a Newly Described Salivary Gland Neoplasm: A Case Report of Microsecretory Adenocarcinoma Presenting in the Parotid Gland.

Introduction: Microsecretory adenocarcinoma (MSA) is a novel entity defined by distinctive histology, a specific immunophenotype, and unique molecular fusion MEF2C::SS18. It occurs mainly in intra-oral minor salivary glands and the skin, with only one reported case affecting the parotid gland. To the best of our knowledge, no cytomorphological features of MSA have been published to date. We report the first case of fine-needle aspiration biopsy (FNAB) cytology of MSA diagnosed in the parotid gland.

Case presentation: A 48-year-old man presented with a 3.5 × 2.5-cm parotid mass. FNAB of the tumour revealed a cellular smear comprising a predominantly epithelial cell population showing luminal differentiation with secretory features and a distinctive background matrix with both myxoid and mucinous qualities. Scattered, but conspicuous multinucleated giant cells were present, a feature not commonly observed in salivary gland aspirates. Histology of the excised tumour revealed classic features of MSA with supportive immunohistochemistry and SS18 break apart fusion detected by fluorescence in situ hybridisation (FISH). Next-generation sequencing confirmed a MEF2C::SS18 gene fusion.

Conclusion: MSA is a rare neoplasm and should be considered in the cytological differential diagnosis of low-grade salivary gland neoplasms. Its unique cytomorphological features should raise the possibility of MSA in salivary gland FNABs. The diagnosis can be established on cellular cell block preparations using immunohistochemistry and FISH or PCR.