Cytological Features of Oncocytic Pleomorphic Adenoma of the Salivary Gland: Using the Milan Classification System to Report 3 Patients

Pleomorphic adenoma (PA) is the most common salivary gland tumour. Pre-operative fine-needle aspiration (FNA) is currently one of the most widely used cytological examination techniques for the diagnosis of salivary gland tumours. Because PA exhibits characteristic cytological features, cytological diagnosis is straightforward in most cases. However, limitations have emerged in certain cases, specifically in cases of oncocytic metaplasia (rare in PA), characterised by rich eosinophilic granular cytoplasm and relatively large nuclei, which can make cytological diagnosis challenging. To date, only two cytological reports of oncocytic PA have been published. The present study retrospectively analysed patients with oncocytic PA of the salivary gland who underwent preoperative FNA to describe the clinicopathological features of oncocytic PA. In addition, the application of the Milan System for Reporting Salivary Gland Cytopathology (MSRSGC) was discussed. The study cohort included 3 patients with parotid gland tumours. The cytological specimens had small and/or large clusters of oncocytic cells containing rich granular cytoplasm, relatively large nuclei, and occasional nucleoli in a myxoid (2 patients) and clear (1 patient) background, with no necrotic material. A review of the cytological features of the presented cases, as well as previously reported cases, indicated that oncocytic PA may be overdiagnosed as carcinoma, especially carcinoma ex pleomorphic adenoma (CXPA) because these cells have relatively large nuclei. The cytological features of oncocytic cells in PA resemble those of salivary duct carcinoma, the most common carcinoma component of CXPA. The absence of necrotic material and high-grade nuclear atypia are important diagnostic features. Furthermore, cytological specimens with atypical oncocytic cells in the PA should be classified as salivary neoplasms in the uncertain malignant potential category of the MSRSGC.