SBRT-LDRT-Cadonilimab for Advanced Gastric, Colorectal and Ovarian Cancers With Peritoneal Metastases

The goal of this clinical trial is to see if high- and low-dose radiotherapy combined with immunotherapy can work in patients with advanced gastric, colorectal, and ovarian cancers with peritoneal metastases, and to learn about the safety of this new combination treatment modality. The main questions they aim to answer are: * Can whole-abdominal low-dose radiotherapy (LDRT) combined with selected-site stereotactic body radiation therapy (SBRT) followed by Cadonilimab control peritoneal lesions and malignant ascites in patients with advanced solid tumors with peritoneal metastases? * Can the novel treatment modality of high- and low-dose radiotherapy combined with immunotherapy produce a survival benefit in patients with advanced gastric, colorectal, and ovarian cancers who have received multiple lines of therapy? * Is the safety profile of this new treatment modality acceptable? Participants will: * Receive stereotactic body radiation therapy (SBRT) to selected sites and low-dose radiotherapy (LDRT) to the whole abdomen, followed by bi-weekly treatment with Cadonilimab until disease progression, death, toxicity intolerance or withdrawal of informed consent. * Receive whole body imaging and laboratory tests every 6-8 weeks to assess the efficacy of tumor treatment.