To enhance the treatment efficacy of rectal cancer liver metastasis through a multidisciplinary approach of radiotherapy, immunotherapy, and chemotherapy, and to provide a new direction for the combination treatment strategy.

Inclusion Criteria:

1. Age 18-75 years, any gender.
2. Pathologically confirmed rectal cancer with liver metastases (stage M1).
3. Karnofsky Performance Status ≥70.
4. Adequate organ function, no contraindications to radiotherapy, or immunotherapy.
5. Microsatellite/mismatch repair status MSS/pMMR.
6. No prior chemotherapy or any other anti-tumor treatment before inclusion.
7. No prior immunotherapy.
8. Ability to comply with the study protocol during the study period.
9. Signed written informed consent.

Exclusion Criteria:

1. Pregnant or lactating women.
2. Pathological diagnosis of signet ring cell carcinoma.
3. History of other malignancies within the past 5 years, except cured skin cancer and cervical carcinoma in situ.
4. Uncontrolled epilepsy, central nervous system disorders, or history of psychiatric disorders that, in the opinion of the investigator, may interfere with signing the informed consent form or affect patient compliance with oral medication.
5. Clinically significant (i.e., active) cardiac disease, such as symptomatic coronary artery disease, New York Heart Association (NYHA) Class II or greater congestive heart failure, or significant arrhythmias requiring drug intervention (see Appendix 12), or history of myocardial infarction within the past 12 months.
6. Organ transplant recipients requiring immunosuppressive therapy and long-term steroid users.
7. Patients with autoimmune diseases.
8. Severe uncontrolled recurrent infections or other severe uncontrolled comorbidities.
9. Subjects with baseline hematological and biochemical parameters not meeting the following criteria: hemoglobin ≥90g/L; absolute neutrophil count (ANC) .≥1.5×10^9/L; platelets ≥100×10^9/L; ALT, AST ≤2.5 times the upper limit of normal; ALP ≤2.5 times the upper limit of normal; serum total bilirubin <1.5 times the upper limit of normal; serum creatinine <1 times the upper limit of normal; serum albumin ≥30g/L.
10. Known deficiency of dihydropyrimidine dehydrogenase (DPD).
11. Allergy to any investigational drug components.