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

Sintilimab: 200mg iv q3w d1

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)

1000mg/m2 Bid po d1-14，q3w，Reduce or discontinue depending on the condition of the subject.

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