PD-L1 22C3 CPS in paired primary breast cancer and lung metastasis.

Purpose: This study aimed to evaluate the expression of PD-L1 22C3 in primary breast cancer and lung metastasis tissues using the Combined Positive Score (CPS) and to investigate the concordance and differences between them.

Methods: Immunohistochemical staining for PD-L1 22C3 was performed on 52 paired cases of primary breast cancer and lung metastases. The CPS was measured and analyzed for comparison.

Results: The PD-L1 positivity rate (CPS ≥ 10) was 13.5% in primary breast cancers and 30.8% in lung metastases. The overall percent agreement (OPA) for PD-L1 CPS between primary tumors and lung metastases was 78.8%, with a positive percent agreement (PPA) of 85.7% and a negative percent agreement (NPA) of 77.8%. The mean CPS was 2.69 ± 6.67 (mean ± SD) in primary breast cancers and 7.67 ± 14.49 (mean ± SD) in lung metastases, with a significantly higher CPS observed in lung metastases (p < 0.001). The primary cause of discordance, where PD-L1 was negative in the primary tumor but positive in the metastasis, was the presence of PD-L1-positive alveolar macrophages predominantly located around or within the metastatic tumor.

Conclusion: PD-L1 22C3 demonstrated approximately 80% concordance between primary breast cancers and lung metastases; however, discordance was mainly attributable to the presence of inherently PD-L1-positive peritumoral and/or intratumoral alveolar macrophages.