mFOLFOX6 + Bevacizumab + PD-1 Monoclonal Antibody Vs. mFOLFOX6 in Locally Advanced pMMR/MSS CRC

Inclusion Criteria:

1. Histologically confirmed adenocarcinoma of the colon or upper rectum.
2. Tumor biopsy immunohistochemical (IHC) identified pMMR, including all of the MSH1,MSH2,MSH6 and PMS2 protein expression and diagnosed as proficient mismatch repair(pMMR), or microsatellite stable (MSS) identified through next-generation sequencing or polymerase chain reaction.
3. Clinical staging of cT4NxM0, with or without positive mesorectal fascia (MRF), and with or without extramural vascular invasion (EMVI); imaging confirms that the lower margin of the tumor is located above the peritoneal reflection (colon or upper rectum).
4. Staging method: All patients must undergo chest, abdominal, and pelvic contrast-enhanced CT, rectal palpation, and high-resolution MRI. Positive perienteric lymph nodes (LNs) are defined as LNs with a short diameter ≥10 mm or LNs exhibiting typical metastatic shape and MRI characteristics. When staging results are contradictory, clinical data must be re-evaluated and confirmed by the central evaluation group. Distant metastases must be excluded through chest and abdominal contrast-enhanced CT and pelvic contrast-enhanced MRI.
5. No symptoms of intestinal obstruction, or obstruction successfully relieved by proximal colostomy.
6. No history of colorectal surgery.
7. No prior chemotherapy or radiotherapy.
8. No history of biopharmaceutical treatments (e.g., monoclonal antibody ), immunotherapy (e.g., anti-PD-1, anti-PD-L1, anti-PD-L2, or anti-CTLA-4 antibodies), or treatment with investigational drugs.
9. Endocrine therapy history: Not restricted.
10. Signed informed consent obtained.

Exclusion Criteria:

1. Arrhythmias requiring anti-arrhythmic treatment (except β-blockers or Digoxin), symptomatic coronary artery disease, myocardial ischemia (myocardial infarction within the past 6 months), or congestive heart failure (CHF) \> NYHA Class II.
2. Severe hypertension that is not well controlled by medication.
3. History of HIV infection or active chronic Hepatitis B or C (with high viral DNA load).
4. Active tuberculosis (TB), ongoing anti-TB treatment, or anti-TB treatment within 1 year prior to trial screening.
5. Other active severe infections as defined by NCI-CTCAE v5.0.
6. Evidence of distant metastasis beyond the pelvic region.
7. Blood dyscrasias or organ dysfunction.
8. History of pelvic or abdominal radiotherapy.
9. Multiple colorectal cancer or multiple primary tumors.
10. Epilepsy requiring treatment (e.g., steroids or anti-epileptic drugs).
11. History of other malignancies within the past 5 years.
12. History of drug abuse, or medical, psychological, or social conditions that could interfere with patient participation or the evaluation of study results.
13. Any active autoimmune disease or a history of autoimmune disease (including but not limited to interstitial pneumonia, uveitis, enteritis, hepatitis, hypophysitis, nephritis, hyperthyroidism, hypothyroidism, and asthma requiring bronchodilators).
14. Administration of any live attenuated vaccine within 4 weeks prior to inclusion.
15. Long-term use of immunosuppressants or systemic/topical corticosteroids (dose \>10 mg/day prednisolone or equivalent).
16. Known or suspected allergy to any study-related drug.
17. Any unstable condition that could compromise patient safety or compliance.
18. Pregnant or breastfeeding women, or women of childbearing potential not using effective contraception.
19. Refusal to provide signed informed consent.