Colon Rectal Cancer Adenocarcinoma

The goal of this clinical trial is to test whether adding targeted radiation plus cadonilimab immunotherapy to standard second-line chemo works better for adults with colorectal cancer only spread to the peritoneum, and check how safe this combined treatment is. Its main research questions are: Does the combined treatment slow cancer growth for a longer time than standard chemo alone? What side effects will participants get from the new combination therapy? Researchers will compare the chemo-radiation-immunotherapy combination against standard chemo alone to see if the new plan shrinks tumors and improves outcomes. Participants will: Receive either the new combined treatment or standard chemo chosen randomly by chance Visit the hospital regularly for drug infusions, scans and physical exams Complete questionnaires about daily quality of life and report any discomfort