Risk of malignancy for biliary cytology based on World Health Organization (WHO) reporting system for pancreaticobiliary cytopathology

ABSTRACT Background The World Health Organization (WHO) classification for pancreaticobiliary cytopathology stratifies risk of malignancy (ROM) and outcome for pancreatic fine‐needle aspirations (FNAs). ROM for biliary cytology, particularly for WHO categories Pan‐low (IV) and Pan‐high (V), has not been well examined. Methods Retrospective data of biliary cytology diagnosed from 2016 to 2019 were collected. WHO classification was assigned. Malignant outcome was determined by histologic correlation or clinical progression/death. Excluding clinical follow‐up <3 months (if no histology), the ROM, odds ratio, performance characteristics, and overall survival were calculated. Results A total of 1056 biliary cytology cases (from 789 patients) were included. Sample types were: 757 (72%) biliary brush/wash, 115 (11%) FNA, and 184 (17%) other. Same‐site histologic correlation was 292 (28%) and histology of metastasis in 124 (12%). The median follow‐up (for benign) was 1431 days. Malignant outcome occurred in 704 ( n = 789, 89%) of patients. Absolute ROM (%) for WHO I to VII by biliary brush/wash and FNA were: 92, 72, 87, 0, 100, 94, 100, and 50, 46, 73, N/A, 100, 80, 100, respectively. A high‐tier (V‐VII) category had significantly higher odds of malignancy regardless of biopsy modality, presence of stent, or autoimmune cholangitis. Conclusion Absolute ROM for biliary lesions is high for WHO I through III and is at least partly because of difficulty in sampling cancers, particularly those extrinsically obstructing the bile duct. The rarity of low‐grade biliary lesions impedes AROM assessment for Pan‐low (WHO IV). The clinical value of biliary Pan‐high (WHO V) remains uncertain because of rarity of high‐grade noninvasive lesions and difficulty in cytology diagnosis.