PD-L1 immunohistochemistry in clinical diagnostics of lung cancer: inter-pathologist variability is higher than assay variability

Abstract

Assessment of programmed cell death ligand 1 (PD-L1) immunohistochemical staining is used for decision on treatment with programmed cell death 1 and PD-L1 checkpoint inhibitors in lung adenocarcinomas and squamous cell carcinomas. This study aimed to compare the staining properties of tumor cells between the antibody clones 28-8, 22C3, SP142, and SP263 and investigate interrater variation between pathologists to see if these stainings can be safely evaluated in the clinical setting. Using consecutive sections from a tissue microarray with tumor tissue from 55 resected lung cancer cases, staining with five PD-L1 assays (28-8 from two different vendors, 22C3, SP142, and SP263) was performed. Seven pathologists individually evaluated the percentage of positive tumor cells, scoring each sample applying cutoff levels used in clinical studies: <1% positive tumor cells (score 0), 1-4% (score 1), 5-9% (score 2), 10-24% (score 3), 25-49% (score 4), and >50% positive tumor cells (score 5). Pairwise analysis of antibody clones showed weighted kappa values in the range of 0.45-0.91 with the highest values for comparisons with 22C3 and 28-8 and the lowest involving SP142. Excluding SP142 resulted in kappa 0.75-0.91. Weighted kappa for interobserver variation between pathologists was 0.71-0.96. Up to 20% of the cases were differently classified as positive or negative by any pathologist compared with consensus score using ≥1% positive tumor cells as cutoff. A significantly better agreement between pathologists was seen using ≥50% as cutoff (0-5% of cases). In conclusion, the concordance between the PD-L1 antibodies 22C3, 28-8 and SP263 is relatively good when evaluating lung cancers and suggests that any one of these assays may be sufficient as basis for decision on treatment with nivolumab, pembrolizumab, and durvalumab. The scoring of the pathologist presents an intrinsic source of error that should be considered especially at low PD-L1 scores.

Expression of programmed cell death protein 1 (PD-1) and its ligand PD-L1 in colorectal cancer: Relationship with sidedness and prognosis

“…Studies regarding the prognostic impact of PD-1 and PD-L1 expression have used different categorical cut-offs, 16-21 some also including staining intensity, and no consensus has yet been reached regarding an optimal prognostic cut-off. For evaluation of tumour cell-specific PD-L1 expression, we applied cut-offs commonly used in clinical studies 46 and the prognostic value of PD-1 + immune cells was validated using the total count. The results from the present study, demonstrating that immune cell-specific PD-1 and PD-L1 expression carries the most evident prognostic value in right-sided CRC, are not likely to be disputed by alternative scoring systems.…”

The reproducibility of PD-L1 scoring in lung cancer: can the pathologists do better?

“…Previous studies have shown that the scoring of immune cells yields low concordance rates and that IHC is probably inadequate for assessment of immune cell expression independent of which assay is selected, with only the possible exception of the SP142 clone, specifically developed for evaluation of tumor associated immune cells (7). Conversely, several studies summarized in Table 1, consistently reported a relatively good agreement for PD-L1 scoring between the pathologists, independent of training and professional education (3,(7)(8)(9)(10).…”

Reproducibility of PD-L1 assessment in non-small cell lung cancer—know your limits but never stop trying to exceed them

“…There is limited data assessing the reproducibility of interpretation and scoring of PD-L1 expression in NSCLC tissue samples and most studies only involve a few pathologists or small numbers of tumors, making it easy to achieve concordance (12)(13)(14)(15)(16). Whereas some studies assessed PD-L1 interpretation reproducibility on resection specimens, only a few studies used smaller sized specimens such as tissue micro-arrays, but none on bronchial or transthoracic biopsies (12)(13)(14)(15)(16).…”

“…Similarly, a recent study showed that up to 20% of the analyzed cases were differently classified as positive or negative by any pathologist compared with the gold standard using the cut point ≥1% TPS, whereas there was better agreement between pathologists using the cut point ≥50% TPS (0-5% of cases) (13).…”

“…Although tremendous efforts have been made to adjust several methodological and biological aspects that may influence the outcome of the PD-L1 assay (i.e., tumor heterogeneity, pre-analytical parameters, assay harmonization, validation; IASLC Atlas of PD-L1 Testing in Lung Cancer), the post-analytical phase, and in particular the interpretation and scoring of PD-L1 expression, may hamper the robustness of a PD-L1 assay for surgical pathologists in their daily practice (12,13).…”