A Single-arm, Phase II Exploratory Study of Sintilimab in Combination With Chemoradiotherapy in Elderly Patients With Locally Advanced Gastric Cancer

This trial is a prospective, single-arm, single-centre, phase II clinical study to explore the efficacy and safety of sintilimab in combination with chemoradiation in subjects over 75 years of age with advanced gastric cancer. Participants will: Take sintilimab (200mg iv q3w d1) combined with chemoradiation. The application of chemotherapy based on investigator's assessment, and if so, S-1 (po d1-d14，q3w； according to body surface area ：\<1.5m\^2 40mg/time；1.5\~1.8m\^2 50mg/time；\>1.8m2 60mg/time) or capecitabine （1000mg/m2 Bid po d1-14，q3w，Reduce or discontinue depending on the condition of the subject.); Radiotherapy：once a day, five times a week, at a dose of 1.8-2 Gy/f, for a total of 45-50.4 Gy. Radiation therapy starts from the first cycle of Sintilimab Injection combined with chemotherapy. Subjects undergo an initial assessment of imaging, physical status, quality of life, and relevant laboratory tests after completion of 3 cycles of sintilimab combination chemotherapy, followed by assessments every 2 months, and after 3 full assessments, assessments every 3 months are initiated.