Checkpoint Inhibitor and Radiotherapy for Recurrent Gastric Cancer (CIRCUIT)

Inclusion Criteria: 1. Unresectable advance or recurrent GC with intolerance or progression after standard treatment (primary and secondary chemotherapy). 2. More than one measurable lesion defined by RECIST guideline version 1.1 in diagnostic imaging (whole-body contrast-enhanced CT or PET-CT) within 14 days before entry, with at least one lesion \>=2 cm. 3. Age: 20 =\< 4. ECOG performance status (PS): 0-2 5. No contraindication for nivolumab (anti-PD-1 antibody) administration. 6. No contraindication for radiotherapy. 7. The most recent laboratory results within 14 days before study entry fulfill the following: WBC ≥3000/μl, neutrophil ≥1500/μl, hemoglobin ≥9.0g/dl, platelets ≥100,000/μl, total bilirubin ≤2.0 times the institutional standard upper limit (ISUL), AST (GOT) and ALT (GPT) ≤3.0 times ISUL (in case with liver metastasis, ≤5.0 times ISUL), serum creatinine ≤1.5 times ISUL or creatinine clearance ≥ 60 ml/min calculated with cockcroft-Gault equation. 8. Expected survival \>=3 months. 9. Written informed consent is obtained from the patient prior to study enrollment. Exclusion Criteria: 1. No tumor lesions that can be irradiated. 2. Metachronous and simultaneous overlapping cancers (excluding intraepithelial cancer of the uterine cervix, fully treated basal cell carcinoma of the skin, and malignant tumors that were treated more than 5 years ago and have not recurred). 3. A history of severe hypersensitivity reactions to other Ab products. 4. Taking immunosuppressive drugs or corticosteroids (prednisone or prednisolone equivalent ≥ 15 mg/day). 5. Active autoimmune diseases or a history of recurrent autoimmune diseases (patients with type-1 diabetes, hypothyroid controllable by hormone replacement therapy, and dermatosis without the need for systemic therapy are eligible). 6. Complications or history of interstitial pneumonia or pulmonary fibrosis diagnosed by imaging studies or clinical findings. 7. Presence of severe disease or medical conditions: severe nutritional deficiencies, transient ischemic attack within 180 days prior to enrollment, cerebral vascular attack within 180 days prior to enrollment, thrombus or thromboembolism within 180 days prior to enrollment, congestive heart failure (NYHA class III or IV), unstable angina, myocardial infarction within 12 months, severe arrhythmias requiring medication, conduction abnormalities such as AV block beyond the second degree, uncontrollable hypertension, liver cirrhosis (Child Class B or higher), mental disorders that may interfere with compliance with this study protocol, unstable diabetes, uncontrolled pericardial fluid, uncontrolled ascites, uncontrolled pleural effusions, diseases requiring anticoagulation therapy (excluding antiplatelet therapy including low-dose aspirin), and systemic infection with treatment. 8. Pregnant or lactating female. 9. Fertile female who are unwilling to use contraception. 10. Fertile male who are not willing to use contraception during study drug administration and for 7 months after study completion (if the partners are fertile females). 11. Prohibited previous treatment: within 56 days of registration; radioactive drugs (except radiopharmaceuticals for examination or diagnostic purposes), within 28 days of registration; corticosteroids (excluding temporary use and predonine or prednisolone equivalent ≤15 mg/day), immunosuppressant drugs, anti-cancer drugs, adhesive treatment of pleura or pericardium, surgery with general anesthesia, and unapproved drugs, within 14 days of registration; surgery with local or superficial anesthesia. 12. Participating in other clinical trials or clinical studies (excludes those without intervention). 13. A positive HIV antigen/Ab test or HTLV-1 Ab test. 14. History of treatment using ONO-4538, anti-PD-1 Ab, anti-PD-L1 Ab, anti-PD-L2 Ab, anti-CD137 Ab, anti-CTLA-4 Ab, or other Ab or drug therapies for T-cell regulation. 15. Determined by the investigator to be ineligible for participation in this study.