Multi-Institutional Study of Pathologist Reading of the Programmed Cell Death Ligand-1 Combined Positive Score Immunohistochemistry Assay for Gastric or Gastroesophageal Junction Cancer

Abstract

The assessment of the expression of programmed cell death ligand-1 (PD-L1) using immunohistochemistry (IHC) has been controversial since its introduction. The methods of assessment and the range of assays and platforms contribute to confusion. Perhaps the most challenging aspect of PD-L1 IHC is the combined positive score (CPS) method of interpretation of IHC results. Although the CPS method is prescribed for more indications than any other PD-L1 scoring system, its reproducibility has never been rigorously assessed. In this study, we collected a series of 108 gastric or gastroesophageal junction cancer cases, stained them using the Food and Drug Administration-approved 22C3 assay, scanned them, and then circulated them to 14 pathologists at 13 institutions for the assessment of interpretative concordance for the CPS system. We found that higher cut points (10 or 20) performed better than a CPS of <1 or >1. We used the Observers Needed to Evaluate Subjective Tests algorithm to assess how the CPS system might perform in the real-world setting and found that the cut points of <1 or >1 showed an overall percent agreement of only 30% among the pathologist raters, with a plateau occurring at 8 raters. The raters performed better at higher cut points. However, the best cut point of <20 versus that of >20 was still disappointing, with a plateau at an overall percent agreement of 70% (at 7 raters). Although there is no ground truth for CPS, we compared the score with quantitative messenger RNA measurement and showed no relationship between the score (at any cut point) and messenger RNA amount. In summary, we showed that CPS shows high subjective variability among pathologist readers and is likely to perform poorly in the real-world setting. This system may be the root cause of the poor specificity and relatively low predictive value of IHC companion diagnostic tests for PD-1 axis therapies that use the CPS system.

Keywords: CPS; GEJ; PD-L1; RNA; closed-system RT-qPCR; gastric cancer; immunohistochemistry.

Figure 1.

Representative images from cohort representing cases with 100% (A, B) and <90% (C, D) agreement among pathologists. 20x image shown with magnified view (inset) (A) example case 1; 100% of pathologists called “<1”. (B) example case 2; 100% of pathologists called “≥1”. (C) example case 3; 79% of pathologists called “<1”. (D) example case 4; 79% of pathologists called “≥1”.

Figure 2.

Percentage of observers who called the cases positive using 3 categories. X-axis shows the individual cases, y-axis is the percent of 14 observers who called a case a certain CPS score. A solid vertical line represents 100% agreement. (A) CPS <1/≥1, (B) CPS <10/≥10, (C) CPS <1, 1–20, <20, and (D) Magnified look at the 10 cases given scores in the 3 ranges (CPS <1, 1–20, <20).

Figure 3.

Percentage agreement between pathologists/observers when looking at all of the cases, and only the cases where all pathologists agreed >90% of the time. Agreement versus (A) years as a practicing pathologist; (B) years signing out PD-L1 cases; (C) volume of PD-L1 cases signed out/ month. For all graphs, linear regressions showed no slopes were significantly non-zero.

Figure 4.

ONEST plots showing overall percent agreement (OPA) between pathologists/observers as a function of the number of observers. Randomly selected curves (n=100) for all possible combinations of observers using 2 category cutoffs for PD-L1 (A) combine positive score (CPS) <1/≥1, (B) CPS <10/≥10, (C) CPS <20/≥20. Red lines indicate the range of the OPA from minimum to maximum at the plateau.

Figure 5.

ONEST plots showing overall percent agreement (OPA) between pathologists/observers using (A) 3 and (B) 5 category cutoffs for PD-L1 CPS.

Figure 6.

Assessment of continuous quantitative mRNA measurement compared to PD-L1 (A) combined positive score (CPS) <1/≥1, (B) CPS <10/≥10, (C) CPS <20/≥20. Mann-Whitney, two-tailed t-tests showed no significant differences.