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 Overall Survival (OS) and Event-free Survival (EFS) in:

• participants with esophageal squamous cell carcinoma (ESCC),
• participants whose tumors express Programmed Death-Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10, and
• all participants.

The primary study hypotheses are that dCRT+ pembrolizumab is better than dCRT + placebo with respect to:

• OS in participants with ESCC,
• OS in participants whose tumors express PD-L1 CPS ≥10,
• OS in all participants,
• EFS in participants with ESCC,
• EFS in participants whose tumors express PD-L1 CPS ≥10, and
• EFS in all participants.