Can precision radiation and immunotherapy team up against abdominal colorectal cancer?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: 1. Histopathologically confirmed colorectal adenocarcinoma. 2. Tumors confirmed pMMR by immunohistochemistry or MSI-L/MSS via gene sequencing. 3. Metastases limited to peritoneum only, no metastases in other organs. 4. No moderate or massive ascites; only trivial physiological effusion without drainage requirement. 5. No symptomatic intestinal obstruction and no obstructive signs on imaging; patients can take food normally. 6. Baseline CT/MRI confirms peritoneal metastases. 7. Positive baseline FAPI PET/CT: at least one peritoneal mass lesion with long diameter ≥1 cm and markedly elevated SUVmax suitable for radiotherapy contouring. 8. At least one measurable lesion per RECIST v1.1 criteria. 9. Progression after first-line standard systemic therapy (FOLFOX/XELOX ± targeted agents); no prior second-line treatment received. 10. Peritoneal recurrence within 1 year after adjuvant fluoropyrimidine-based chemotherapy. 11. No moderate or massive ascites; only trivial physiological effusion without drainage requirement. 12. Voluntarily sign written informed consent and comply with scheduled study visits and treatment procedures. Exclusion Criteria: 1. Prior exposure to any anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CTLA-4 antibody or other agents targeting T-cell co-stimulatory/checkpoint pathways. 2. Moderate or massive ascites requiring clinical intervention. 3. Partial or complete symptomatic intestinal obstruction. 4. Pregnant or breastfeeding women. 5. History of high-dose abdominal radiotherapy that prevents re-irradiation. 6. Diffuse miliary peritoneal metastases on FAPI PET/CT without definable target volume for radiotherapy. 7. Active autoimmune diseases (systemic lupus erythematosus, rheumatoid arthritis, etc.); systemic corticosteroids or other immunosuppressants required within 14 days before enrollment; severe underlying vital organ disease judged unsuitable for immunotherapy by investigators. 8. Poor compliance or other conditions judged inappropriate for study participation by investigators.