The diagnostic accuracy of pleomorphic adenoma in fine needle aspiration: Features leading to false positive and false negative diagnoses and the utility of cell blocks

Objective: Fine needle aspiration (FNA) is a well-established tool in preoperative diagnosis of salivary gland lesions with diagnostic accuracy of 90%. Pleomorphic ade-noma (PA) is the most common salivary gland tumor comprising 45%-74% of all sali-vary gland tumors with FNA diagnostic accuracy of 89.5%-96.2%. The aim of the present study was to determine and analyze potential cytomorphological pitfalls and evaluate the diagnostic accuracy in FNA diagnosis of PA.Methods: Salivary gland specimens with both cytological and histological diagnoses were searched over a 10-year-period (2009-2018) from a laboratory information system of Pathology Department, Fimlab Laboratories, Tampere and matched to determine concordant and discordant PA cases. Sufficient material in histological and cytological sample was found in 401 cases. In 218 cases (54.4%) diagnosis was true-negative PA, in 169 cases (42.1%) diagnosis was true-positive PA and there were 14 discordant cases: 4 false-positive cases and 10 false-negative cases. False-negative cases were reclassified and subgrouped according to The Milan System for Reporting Salivary Gland Cytopathology (MSRSGC).Results: Cytomorphologically, cell type predominance was more often myoepithelial in true-positive cases (65%) and epithelial both in false-negative (70%, p = .007) and false-positive cases (75%, p = .027). Well-formed ducts were present in cytology in all true-positive cases (p < .001). Only 10% of true-positive cases did not show any matrix in cytology (p < .001). Nuclear changes were common in false-negative cases (80%, p = .002) and false-positive cases (75%, p = .003). Beneficial cell block (CB) was more common in true-positive cases (85%) than in false-negative cases (50%, p = .041) or in false-positive cases (50%, p = .116) and a lack of beneficial CB led more often to a false diagnosis (70% false diagnosis without beneficial CB versus 29% false diagnosis with beneficial CB).Conclusion: The present study showed diagnostic accuracy of 96.5% for FNA in PA diagnosis. Sensitivity, specificity, positive predictive value and negative predictive value were 94.4%, 98.2%, 97.7%, and 95.6%, respectively. The benefit of CBs was more evident in true-positive cases (85%).