The purpose of this study is to assess the efficacy and safety of treatment with definitive chemoradiotherapy (dCRT) + pembrolizumab (MK-3475) compared to treatment with dCRT + placebo with respect to Event-free Survival (EFS) and Overall Survival (OS) in: * participants whose tumors express Programmed Death-Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10 * participants whose tumors express PD-L1 CPS ≥1 * all participants The primary study hypotheses are that dCRT+ pembrolizumab is better than dCRT + placebo with respect to: * EFS in participants whose tumors express PD-L1 CPS ≥10 * EFS in participants whose tumors express PD-L1 CPS ≥1 * EFS in all participants * OS in participants whose tumors express PD-L1 CPS ≥10 * OS in participants whose tumors express PD-L1 CPS ≥1 * OS in all participants
Participants receive pembrolizumab or placebo PLUS one of two chemotherapy regimens PLUS radiation therapy for up to approximately one year. The chemotherapy regimens are either: * FP (5-fluorouracil \[5-FU\] + cisplatin) or * FOLFOX (5-FU + oxaliplatin + leucovorin or levoleucovorin).
IV infusion
IV infusion
IV infusion
IV infusion
external radiation
IV infusion
IV infusion
IV infusion
Buenos Aires, Buenos Aires F.D., Argentina
Ciudad de Buenos Aires, Buenos Aires F.D., Argentina
Buenos Aires, Argentina
Buenos Aires, Argentina