This pilot phase I trial aims to investigate the efficacy and safety of high- and low-dose radiotherapy combined with programmed cell death-1 (PD-1) inhibitors in microsatellite stable (MSS) metastatic colorectal cancer (mCRC) that have failed second-line immunotherapy or above.

This pilot phase I trial aims to investigate the efficacy and safety of high- and low-dose radiotherapy combined with programmed cell death-1 (PD-1) inhibitors in microsatellite stable (MSS) metastatic colorectal cancer (mCRC) that have failed second-line immunotherapy or above. 30 participants will be enrolled in this study. All will take part at Jiangsu Cancer Hospital.

Inclusion Criteria:

1. Patients with microsatellite stable colorectal liver metastases and failure with second-line or above therapy, and no subsequent standard treatment regimen.
2. Patients with an ECOG score of 0 or 1, and an expected survival period of ≥6 months.
3. During the study, they are willing to follow the arrangement and not use other systemic anti-tumor drugs such as chemotherapy, targeted, Chinese herbal medicine, and proprietary Chinese medicine.
4. 18-70 years old, no gender limit.

Exclusion Criteria:

1. Those with a history of severe immediate allergy to the drugs used in this study.
2. Cancer patients who require urgent surgical intervention, such as high-risk pathological fractures, life-threatening bleeding symptoms, etc.
3. Any of the following conditions in the 6 months before screening: myocardial infarction, severe/unstable angina, coronary artery/peripheral artery bypass graft, symptomatic congestive heart failure, cerebrovascular accident, transient cerebral ischemia Onset or symptomatic pulmonary embolism. Patients with known coronary artery disease, congestive heart failure that does not meet the above criteria or left ventricular ejection fraction <50% must adopt an optimized and stable medical plan determined by the treating doctor. If appropriate, you can consult a cardiologist.
4. Patients with active infection requiring systemic treatment.