Radiotherapy + Chemoimmunotherapy Followed by Surgery in Patients With Limited Metastatic Gastric or GEJ Cancer

The goal of this clinical trial is to evaluate the efficacy and safety of radiotherapy combined with chemotherapy and anti-PD-1 immunotherapy followed by surgery for the primary and metastatic lesions in patients with limited metastatic gastric or gastroesophageal junction adenocarcinoma. The main questions it aims to answer are: 1) If the multimodal treatment which includes anti-PD-1 immunotherapy and local therapies will improve the survival of this group of patients. 2) If the multimodal treatment which includes anti-PD-1 immunotherapy and local therapies can be performed safely in this group of patients. Participants will receive short course hypofractionated radiotherapy (HFRT) for the primary lesion, HFRT or stereotactic body radiotherapy (SBRT) for metastatic lesions, combined with systemic chemotherapy and anti-PD-1 immunotherapy. For patients with HER2-positive cancer (defined as IHC 3+ or 2+/ISH+), trastuzumab is used along with chemotherapy and anti-PD-1 antibody. Then, surgical resections of primary and metastatic lesions are performed as much as possible. For patients who need a widely invasive surgical approach or are inoperable, local ablative therapies such as radiofrequency ablation (RFA) and microwave ablation (MVA) can be alternatives. For patients undergoing surgical resections, postoperative treatment includes chemotherapy, which is determined by the researcher, and PD-1 antibody, which will be maintained until one year after surgery.