The programmed death ligand 1 immunohistochemistry 22C3 pharm DX assay （PD-L1/22C3） is commonly used for assessing PD-L1 expression in non-small-cell lung cancer. Although various sample types have been used for the PD-L1 assay, the feasibility of the PD-L1/22C3 assay in clinical practice remains undefined. At Showa University Hospital, 270 patients diagnosed with primary lung cancer and 271 pathological specimens were assessed. The overall failure rate of the PD-L1/22C3 assay, tumor proportion score （TPS） distribution, and clinical characteristics were retrospectively reviewed. Efficacy, including objective response rate, progression-free-survival, and overall survival, following pembrolizumab monotherapy for patients with high PD-L1 expression and anti-PD-1/PD-L1 treatment for previously treated patients were also retrospectively analyzed. The overall failure rate for the PD-L1/22C3 assay was 3.0%. PD-L1 expression classified by TPS＜1%, 1-49%, and≥50% was 31%, 33%, and 33%, respectively. Thirty-one patients with high PD-L1 expression （TPS≥50%） received first-line pembrolizumab monotherapy, which exhibited high efficacy and outcome, irrespective of the diagnostic procedure. In 65 patients, anti-PD-1/PD-L1 monotherapy used as second- or further-line treatment showed moderate efficacy, irrespective of the diagnostic procedure and the period between tumor acquisition and PD-L1 assay. However, PD-L1 positivity did not affect clinical outcome. The PD-L1/22C3 assay is feasible in a clinical setting because of its low failure rate and it is a good predictor of pembrolizumab efficacy. For previously treated patients, prediction of the effectiveness of anti-PD-1/PD-L1 treatment based on PD-L1 expression should be considered.
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