Warthin's tumour diagnostic outcome in the Milan System era and cytomorphological pitfalls.

Background: Warthin´s tumor (WT) is the second most common salivary gland neoplasm. With classic cytomorphological features of WT, the diagnostic accuracy is over 95%. WT is usually categorized as benign neoplasm according to the Milan System for Reporting Salivary Gland Cytopathology (MSRSGC).

Methods: Database search at the Department of Pathology, Fimlab Laboratories, Tampere, Finland revealed 146 WTs during a 10-year period (January 1st, 2013-December 31st, 2022). Diagnostic accuracy was calculated for the entire study period, and the study period divided in half to pre-MSRSGC years (2013-2017) and MSRSGC years (2018-2022). In addition, a separate cytomorphology analysis of false-negative cases that were classified according to the MSRSGC, was performed.

Results: Diagnostic accuracy was 96.4%, sensitivity was 68.5%, and specificity was 99.8%. Sensitivities and specificities were almost equal during the pre-MSRSGC years and the MSRSGC years. The number of true-positive cases was 113. Fifty-five cases (52 false-negative and three false-positive cases) were not accurately diagnosed. Risk of malignancy and risk of neoplasm were 0.0% and 98.3% of cases that were cytologically diagnosed as WT. Cytomorphological analysis showed that lack of papillae, the presence of small groups, and cystic degeneration led to false diagnoses. In addition, necrosis and diffuse hypercellularity increased the suspicion of malignancy and led to classification of fine-needle aspirations as salivary gland neoplasm of uncertain malignant potential.

Conclusion: The MSRSGC is useful in WT diagnostics, and it improves communication between cytopathologists and clinicians. In this study, the most useful cytomorphological features that led to accurate WT diagnoses was papillary architecture in cell block specimens and the most significant pitfall was necrosis followed by diffuse hypercellularity.