Re-Categorization and Risk of Malignancy of Atypical Effusions According to the International System for Reporting Serous Fluid Cytopathology (TIS): An Institutional Experience.

Background: Serous effusion fluids serve as crucial cytological materials in diagnosing the underlying causes of fluid accumulation in various body cavities. The International System for Reporting Serous Fluid Cytopathology (TIS) outlined atypia of undetermined significance (AUS) and suspicious for malignancy (SFM) as two intermediate categories for atypical fluids.

Materials and methods: In our study, cases diagnosed as "atypical" by an experienced cytopathologist in the Hacettepe University Department of Pathology, between 2014 and 2023, were re-categorized according to TIS. The risk of malignancy (ROM) for AUS and SFM categories was calculated. Additionally, cases were re-evaluated according to the 5-tier categorical system by two observers with different level of experience.

Results: A total of 3501 effusion fluids were included in the study, evaluated between 2014 and 2023. 50.7% of the cases were male, and 49.3% were female. 55.2% of the cases were pleural fluid, 41.5% were peritoneal fluid, and 3.3% were pericardial fluid. Two hundred sixty five cases (7.6%) were non-diagnostic, 2160 (61.7%) were negative for malignancy, 111 (3.2%) were AUS, 83 (2.4%) were SFM, and 883 (25.2%) were malignant (M). ROM calculated 27% for AUS, 49% for SFM 49%. The interobserver agreement for AUS was 46% with kappa value of 0.162, and 42% for SFM with kappa value of 0.188.

Conclusion: The differentiation of atypical cases into AUS and SFM is effective in determining the ROM. The interobserver agreement tends to be lower in intermediate categories compared to others, emphasizing the potential significance of clinical expertise in diagnosis.