Discordance of PD-L1 status between primary and metastatic breast cancer: A systematic review and meta-analysis

•This is a systematic review and meta-analysis of PD-L1 expression in breast cancer.•PD-L1 expression rates were higher in primary tumors than in metastases.•Discordance of PD-L1 expression was more common in immune than tumor cells.•Discordance between PD-L1 status in primary tumor and metastasis was bi-directional.•These results highlight the need to obtain metastatic biopsies. Programmed cell death ligand 1 (PD-L1) expression is predictive for benefit from immunotherapy in several human malignancies including triple negative breast cancer. Lower positivity rates but a larger relative benefit from atezolizumab has been implied when PD-L1 status is assessed at metastatic sites. We aimed to study the discordance of PD-L1 expression between primary tumor and metastasis in breast cancer due to its potential clinical utility. Cochrane Library, Embase, Medline and Web of science were searched for studies reporting on PD-L1 expression in primary and metastatic breast cancer, followed by data extraction. Outcomes included pooled PD-L1 positivity rates in tumor cells, immune cells or both in primary tumor and metastasis, PD-L1 discordance between matched primary tumors and metastasis and direction of discordance. Of 2552 identified entries following de-duplication, 20 studies fulfilled the predefined inclusion criteria. Pooled PD-L1 positivity rate was higher in primary tumors compared to metastasis when assessed in immune cells (51.2% vs 37.1% p