Application of the World Health Organization Reporting System for Lung Cytopathology: Risk of Malignancy and Overall Survival in Fine-Needle Aspiration Biopsies

Introduction: The World Health Organization (WHO) reporting system for lung cytopathology provides a 5-tiered diagnostic framework. However, validation of the associated risk of malignancy (ROM) and clinical outcomes remains limited. This study evaluated the performance of the WHO reporting system by assessing ROM and overall survival (OS) in lung fine-needle aspiration biopsies (FNAB) specimens. Methods: A retrospective review of the institutional pathology database identified patients who underwent endobronchial ultrasound-guided transbronchial FNAB (EBUS-TBFNAB) or transthoracic CT-guided FNAB for lung lesions between January 2015 and June 2022. All cases were recategorized according to the WHO reporting system. Absolute ROM was determined by histologic and/or clinical follow-up of at least 6 months and compared with published studies that used the WHO and other international reporting systems. OS rates were analyzed across diagnostic categories. Results: A total of 1,038 FNAB cases were included. The ROM for each WHO diagnostic category was as follows: insufficient/inadequate/non-diagnostic, 19%; benign, 9%; atypical, 33%; suspicious for malignancy, 84%; and malignant; 100%. These findings demonstrated overall concordance with prior studies, with comparatively lower ROMs in the insufficient/inadequate/non-diagnostic, benign, and atypical categories, and upper-range ROMs in the suspicious for malignancy and malignant categories. Immunocytochemistry (ICC) and molecular testing performed on available cell block materials supported tumor subtyping and therapeutic decision-making. Conclusions: Application of the WHO reporting system significantly improves stratification of ROM and OS across diagnostic categories in lung FNAB specimens. Inclusion of a benign category and reclassification of low-grade carcinomas as malignant enhance diagnostic precision and support optimized patient management.