Condition: Gastroesophageal Junction Carcinoma

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