Application of the International System for Reporting Serous Fluid Cytopathology for Risk of Malignancy Assessment: An Institutional Experience

Summary: Introduction: The International System for Reporting Serous Fluid Cytopathology (TIS) provides a standard terminology for reporting serous fluid cytology. It defines the diagnostic criteria, helps with the communication between clinicians and pathologists and with setting clinical management guidelines according to the risk of malignancy (ROM). TIS categories include I- Non-diagnostic (ND, II- Negative for malignancy (NFM), III- Atypia of undetermined significance, IV- Suspicious for malignancy (SFM), V- malignant (MAL). The aim of this study is to reclassify pleural, peritoneal, and pericardial effusions based on the TIS criteria and to assess the ROM for each diagnostic category at our institution. Materials and Methods: With Internal Board Review approval, we retrieved from our electronic cytopathology records all pleural, peritoneal/abdominal and pericardial effusions received between 01/2022 and 12/2022. These were reclassified following the TIS criteria. Cytohistological correlation was performed in cases with available histology to assess the ROM for each TIS category. If surgical follow-up was not available, clinical-radiologic data, and/or positive repeat cytology fluid within 6 months period were used for assessment. Results: A total of 978 serous fluids (560 abdominal/ peritoneal, 392 pleural, 26 pericardial) were received in the cytopathology laboratory. Abdominal/peritoneal effusions were the most common fluids received (57%), followed by pleural (40%) and pericardial (2.6%). Out of the 978 fluids, 246 (25%) were positive for malignancy (25%), with adenocarcinoma of the lung as the most common malignancy (22%). In our cohort, 17 (1%) cases were non-diagnostic; 824 (84%) cases were negative for malignancy; 35 (3%) were atypia of undetermined significance; 2 (0.2%) were suspicious for malignancy and 100 (10%) were malignant after reclassification. Calculated ROM was 0% for ND, 14% for NFM, 65% for AUS, 100% for SFM and 100% for MAL cases (Table 1). Conclusions: Applying the TIS classification in reporting on serous fluids could improve the consistency of terminology among different institutes. The results of calculated ROM in our institute are in accordance with the TIS implied ROM.