The goal of this clinical trial is to learn whether a high- and low-dose radiotherapy regimen followed by anti-angiogenic TKI and anti-PD-1 antibody therapy works to treat advanced metastatic colorectal cancer. It will also evaluate long-term survival outcomes and explore potential biomarkers associated with tumor response and immune modulation.

The main questions it aims to answer are:

Does the high- and low-dose radiotherapy regimen followed by sequential anti-angiogenic TKI and anti-PD-1 therapy improve the objective response rate (ORR) in patients with advanced metastatic colorectal cancer?

What are the disease control rate (DCR) and survival outcomes following this treatment strategy?

Are tumor response and long-term survival associated with specific biomarkers related to systemic immune modulation induced by radiotherapy to different metastatic organs?

How does this radiotherapy pattern affect tumor immune infiltration in metastatic lesions?

This is a single-arm, single-center, prospective Phase II clinical study designed to evaluate the efficacy of a high- and low-dose radiotherapy regimen targeting metastatic lesions followed by sequential anti-angiogenic TKI and anti-PD-1 antibody therapy in patients with advanced metastatic colorectal cancer.

Participants will:

Receive high- and low-dose radiotherapy to metastatic lesions. Subsequently receive anti-angiogenic TKI combined with anti-PD-1 antibody therapy.

Undergo regular clinical assessments and imaging evaluations to determine tumor response.

Provide blood and tumor samples for biomarker and immune infiltration analysis. Be followed for survival outcomes and disease progression.

Inclusion Criteria:

1. Ability to understand and willingness to sign a written informed consent form.

2. Age 18-80 years, male or female.

3. Histologically confirmed microsatellite stable (MSS) / proficient mismatch repair (pMMR) colorectal adenocarcinoma.

4. Clinical stage IV disease confirmed by systemic imaging. At least two measurable metastatic lesions per RECIST 1.1, including:

   At least one lesion ≥2 cm in diameter and at least one lesion <2 cm in diameter;

   For lesions within the same organ, at least one lesion ≥2 cm and one lesion <2 cm must be located within independent, non-overlapping radiation fields;

   For lesions in different organs, high-dose and low-dose radiotherapy should preferably be delivered to lesions in different organs.

   Second-line or later treatment setting, defined as disease progression after prior platinum- and irinotecan-based chemotherapy, including:

5. Failure after first-line triplet chemotherapy containing platinum and irinotecan; Or failure after first-line platinum-based chemotherapy followed by second-line irinotecan-based chemotherapy.

6. Ability to swallow oral medication.

7. ECOG performance status 0-1.

8. Adequate organ function as defined by laboratory criteria.

Exclusion Criteria:

Diffuse miliary liver or lung metastases, or bulky metastatic lesions ≥10 cm in diameter.

Presence of brain metastases.

1. History of severe hypersensitivity to monoclonal antibodies or anti-angiogenic agents.

2. Prior exposure to immune checkpoint inhibitors (including anti-PD-1, anti-PD-L1, anti-CTLA-4 antibodies), immune checkpoint agonists (e.g., targeting ICOS, CD40, CD137, GITR, OX40), or any prior cancer immunotherapy.

3. Active autoimmune disease or history of autoimmune disease, including but not limited to interstitial pneumonitis, colitis, hepatitis, hypophysitis, vasculitis, nephritis, hyperthyroidism, or hypothyroidism (patients on stable hormone replacement therapy may be eligible). Patients with psoriasis or childhood asthma/allergy that has completely resolved and requires no adult intervention may be eligible; patients requiring bronchodilator therapy are excluded.

4. History of immunodeficiency, including positive HIV test, congenital or acquired immunodeficiency, prior organ transplantation, or allogeneic bone marrow transplantation.

5. Uncontrolled cardiovascular disease, including:

   NYHA class II or higher heart failure;

6. Unstable angina;

7. Myocardial infarction within 1 year;

8. Clinically significant uncontrolled supraventricular or ventricular arrhythmias.

9. Severe infection (CTCAE grade >2) within 4 weeks prior to first study treatment, including severe pneumonia, bacteremia, or infection requiring hospitalization. Active pulmonary inflammation on baseline imaging, signs or symptoms of infection within 14 days before treatment requiring systemic antibiotics (except prophylactic antibiotics), or active tuberculosis (current or within 1 year without adequate treatment).

10. Active hepatitis B (HBV DNA ≥2000 IU/mL) or active hepatitis C (positive HCV antibody with detectable HCV RNA above the lower limit of detection).

11. Diagnosis of another malignancy within 5 years prior to first study treatment, except for malignancies with low risk of metastasis or death (5-year survival rate >90%), such as adequately treated basal cell carcinoma, squamous cell carcinoma of the skin, or carcinoma in situ of the cervix.

12. Pregnant or breastfeeding women.