Study of Adjuvant Chemotherapy With or Without PD-1 Inhibitors and Chemoradiotherapy in Resected pN3 Gastric (G) or GEJ Adenocarcinoma

This study tests whether adding an immunotherapy drug (PD-1 inhibitor) to standard chemotherapy and chemoradiotherapy after surgery improves outcomes for patients with stomach (gastric) or gastroesophageal junction cancer that had extensive lymph node involvement (more than 7 positive nodes) at the time of surgery. **You may be eligible if...** - You are in good general health (ECOG 0–1) with expected survival more than 6 months - You had a complete D2/R0 surgical removal of your stomach or GEJ cancer - Your cancer had more than 7 positive lymph nodes (stage pN3) with no distant spread - Your blood counts, liver, and kidney function meet required standards - You are willing to provide blood and tissue samples **You may NOT be eligible if...** - You had peritoneal spread of cancer (with some exceptions) - You received any anti-cancer treatment before surgery - You have an active autoimmune disease requiring systemic treatment - You have known HIV, active tuberculosis, active hepatitis B or C - You are pregnant or breastfeeding - You have had another cancer in the past 5 years (except treated basal cell skin cancer or carcinoma in situ) Talk to your doctor to see if this trial is right for you.