Node-Sparing Short-Course Radiation With CAPOX and Sintilimab for MSS Locally Advanced Colon Cancer: A Randomized, Prospective, Multicenter Study

Inclusion Criteria:

* 1\. Patients willing to undergo neoadjuvant treatment. 2. Age ≥ 18 years. 3. Tumor confirmed by colonoscopy and enhanced abdominal CT to be ≥ 12 cm from the anal verge.

  4\. Histologically diagnosed adenocarcinoma; genetic testing indicates MSI-L or MSS, or immunohistochemistry from tumor biopsy shows pMMR (all four proteins-MSH1, MSH2, MSH6, and PMS2-are positive).

  5\. Clinical staging by enhanced abdominal CT evaluates as cT3-4N0-2M0. 6. ECOG performance status score of 0-1. 7. No prior treatment with anti-tumor, immunotherapy, or abdominal radiation therapy before enrollment.

  8\. Blood test results (without transfusion within 14 days and no use of granulocyte colony-stimulating factor or other hematopoietic stimulators within 7 days before the lab test):
  1. White blood cell count ≥ 3.5 × 10\^9/L, absolute neutrophil count ≥ 1.5 × 10\^9/L, platelet count ≥ 100 × 10\^9/L, hemoglobin concentration ≥ 9 g/dL;
  2. Liver function tests (bilirubin ≤ 1.5 × ULN; AST and ALT ≤ 5 × ULN);
  3. Renal function (serum creatinine ≤ 1.5 × ULN or creatinine clearance (CCr) ≥ 50 mL/min);
  4. Coagulation (INR ≤ 1.5 × ULN, PT and APTT ≤ 1.5 × ULN);
  5. Thyroid function: TSH ≤ upper limit of normal (ULN); if abnormal, FT3 and FT4 levels must be evaluated, and if FT3 and FT4 are normal, the patient is eligible.

     9\. Voluntary participation with a signed informed consent form.

     Exclusion Criteria:
* 1\. History of other malignancies within the past 5 years. 2. Patients with metastases in other locations (stage IV). 3. Patients with MSI-H or dMMR. 4. Patients with conditions such as bowel obstruction, perforation, or bleeding requiring emergency surgery.

  5\. Known allergy to the study drug or any of its excipients. 6. Patients with any unstable systemic diseases, including but not limited to severe infections, uncontrolled diabetes, uncontrolled hypertension, unstable angina, cerebrovascular accidents or transient ischemic attacks, myocardial infarction, congestive heart failure, or severe illnesses requiring medication (such as arrhythmias, liver, kidney, or metabolic diseases) that are life-threatening.

  7\. History of active autoimmune diseases requiring systemic treatment (e.g., disease-modifying drugs, corticosteroids, or immunosuppressants) within the last 2 years. Replacement therapies (e.g., thyroid hormone, insulin, or physiological corticosteroid replacement for adrenal or pituitary insufficiency) are not considered systemic treatments.

  8\. Known history of HIV infection or acquired immunodeficiency syndrome (AIDS). 9. Receipt of any investigational drug (including immunotherapy) or participation in another interventional clinical study within 30 days before screening.

  10\. Pregnant or breastfeeding women, or women planning to become pregnant or breastfeed during the study; men or women unwilling to use effective contraception during the study.

  11\. Vulnerable populations, including those with mental illness, cognitive impairment, or critically ill patients.

  12\. Any other conditions deemed inappropriate for participation by the investigator.