Fine‐Needle Aspiration of Salivary Gland Tumors With Low‐Grade Oncocytic Features: Focusing on Oncocytoma Versus Oncocytic Mucoepidermoid Carcinoma

BACKGROUND: Salivary gland fine-needle aspirates (FNAs) with low-grade oncocytic features are diagnostically challenging due to the substantial morphologic overlap between benign oncocytic tumors and their malignant mimickers. This study evaluates the diagnostic discriminators, performance, and risk of malignancy of these lesions using the Milan System for Reporting Salivary Gland Cytopathology (MSRSGC). METHODS: We retrospectively reviewed salivary gland FNA cases (2021-2025) categorized as Milan IVa: Benign neoplasm with oncocytic features, including Warthin tumor and oncocytoma/nodular oncocytic hyperplasia (NOH), and Milan IVb: Salivary gland neoplasm of uncertain malignant potential (SUMP) with oncocytic features, with surgical follow-up. Cytomorphological analysis was performed on liquid-based cytology (LBC), direct smears, and cell blocks. Features including architectural patterns, cytological details, and background were correlated with histology. RESULTS: Forty-nine cases were included. All 43 cases diagnosed as Milan IVa were confirmed as Warthin tumor or oncocytoma/nodular oncocytic hyperplasia. Warthin tumor shows cohesive, flat clusters of oncocytes along with a cystic debris-like background, while oncocytoma/NOH shows compact globoid clusters of oncocytes with a "bunch of grapes" pattern and clean background. In the six Milan IVb cases, follow-up histology included oncocytic mucoepidermoid carcinoma (OMEC, n = 2), oncocytoma/nodular oncocytic hyperplasia (n = 2), oncocytic lipoadenoma (n = 1), and spindle cell lipoma (n = 1). The OMEC cases form discohesive clusters and single cells. Cell block preparations revealed focal glandular structures and rare intracytoplasmic mucin. CONCLUSION: The MSRSGC effectively risk-stratifies low-grade oncocytic salivary gland lesions. While cohesive, organoid architecture supports a benign diagnosis, the presence of isolated cells and discohesive clusters is better classified as SUMP. Cell block preparation is useful for identifying mucin and glandular differentiation.