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Flow diagram for the selection of eligible studies. RCT: randomised controlled trials; CT: chemotherapy; PD-1: programmed death–1 receptor inhibitor; PD–L1: programmed death ligand–1 inhibitor; CTLA–4: cytotoxic T lymphocyte associated antigen inhibitor; n: number; *: one study involved two groups: PD-1+CTLA–4 vs. PD–1 vs. CT (group A) and PD–1+CTLA–4 vs. PD–1+CT vs. CT (group B).
