High Interobserver Variability among Pathologists Using Combined Positive Score to Evaluate PD-L1 Expression in Gastric, Gastroesophageal Junction and Esophageal Adenocarcinoma.

Reliable, reproducible methods to interpret programmed death ligand-1 (PD-L1) expression on tumor cells (TC) and immune cells (IC) are needed for pathologists to inform decisions associated with checkpoint inhibitor therapies. Our international study compared interpathologist agreement of PD-L1 expression using combined positive score (CPS) under standardized conditions on samples from patients with gastric/gastroesophageal junction/esophageal adenocarcinoma. Tissue sections from 100 adenocarcinoma pre-treatment biopsies were stained in a single laboratory using the PD-L1 IHC 28-8 and 22C3(Agilent) pharmDx immunohistochemical assays. PD-L1 CPS was evaluated by 12 pathologists on scanned whole slide images of these biopsies before and after a 2-hour CPS training session by Agilent. Additionally, pathologists determined PD-L1 positive TC, IC, and total viable TC on a single tissue fragment from 35 of 100 biopsy samples. Scoring agreement among pathologists was assessed using the intraclass correlation coefficient (ICC). Interobserver variability for CPS for 100 biopsies was high with only fair agreement among pathologists both pre- (range 0.45 to 0.55) and post-training (range 0.56 to 0.57) for both assays. For the 35 single biopsy samples, poor/fair agreement was also observed for the total number viable TC (ICC 0.09), number of PD-L1 positive IC (ICC 0.19), number of PD-L1 positive TC (ICC 0.54), and calculated CPS (ICC 0.14), while calculated TC score (positive TC/Total TC) showed excellent agreement (ICC 0.82). Retrospective histologic review of samples with the poorest interpathologist agreement revealed (1) ambiguous identification of positively staining stromal cells, (2) faint or variable intensity of staining, (3) difficulty in distinguishing membranous from cytoplasmic tumor staining, and (4) cautery and crush artifact, as possible confounding factors. These results emphasize the need for objective techniques to standardize the interpretation of PD-L1 expression when using the CPS methodology on gastric/gastroesophageal junction cancer biopsies to accurately identify patients most likely to benefit from immune checkpoint inhibitor therapy.