Supplementary Material for: PD-L1 in lung cytology: the path for standardization

Background: The advent of PD-1/PD-L1 inhibitors has revolutionized lung cancer treatment, necessitating accurate PD-L1 immunohistochemical (IHC) assessment. While standardized for formalin-fixed paraffin-embedded (FFPE) histological samples, PD-L1 testing on cytology remains challenging. This review aims to address the complexities of PD-L1 IHC in cytology, focusing on validation guidelines, quality assessment, cyto-histological correlation, and interobserver variability. Summary: This review synthesizes current guidelines and research on PD-L1 IHC in cytology; in particular, recent College of American Pathologists (CAP) guidelines emphasize the necessity for rigorous validation, particularly for non-formalin-fixed specimens. As far as cyto-histological concordance studies is concerned, the review of 48 original articles reveal significant variability in PD-L1 expression, with concordance rates ranging from 54-100% at the 1% cutoff and 82-100% at the 50% cutoff. Finally, interobserver variability, particularly in the 1-49% PD-L1 expression range, further complicates accurate assessment. The review also discusses the challenges associated with quality assessment in cytology, including the lack of standardized control materials and external quality assessment (EQA) programs specifically tailored for cytological samples. Conclusion: PD-L1 testing on cytology presents significant challenges, including validation complexities, quality control limitations, and interobserver variability. Standardized guidelines and rigorous validation are essential to ensure accurate and reliable PD-L1 assessment in cytological specimens.