The patient (T4N0 or low-risk stage III) will be randomly assigned to either the control group (FOLFOX/CAPOX for 3 months) or the intervention group (FOLFOX/CAPOX for 6 months or FOLFOX/CAPOX for 3 months followed by FOLFIRI/CAPIRI for 3 months) to receive adjuvant therapy. Venous blood samples (8-16 mL) will be collected at 1 month, 3 months, and 6 months after surgery for dynamic monitoring of plasma ctDNA.

The patient (T4N0 or low-risk stage III) will be randomly assigned to either the control group (FOLFOX/CAPOX for 3 months) or the intervention group (FOLFOX/CAPOX for 6 months or FOLFOX/CAPOX for 3 months followed by FOLFIRI/CAPIRI for 3 months) to receive adjuvant therapy. Venous blood samples (8-16 mL) will be collected at 1 month, 3 months, and 6 months after surgery for dynamic monitoring of plasma ctDNA.

Inclusion Criteria:

1. Age ≥18 years, regardless of sex;
2. Eastern Cooperative Oncology Group (ECOG) performance status of 0-2, with an expected survival of >3 months;
3. Histologically confirmed postoperative pTNM stage high-risk stage II colorectal cancer;
4. Positive ctDNA status at 1 month after surgery;
5. Expected survival of >12 months;
6. Ability to understand and willingness to sign a written informed consent form (personally or via a legally authorized representative/guardian), indicating that the subject understands the study objectives and required procedures and agrees to participate.

Exclusion Criteria:

1. Receipt of neoadjuvant therapy prior to surgery;
2. Blood transfusion during surgery or within 2 weeks prior to surgery;
3. Pregnant or breastfeeding women, or individuals of reproductive potential who are not using adequate contraception;
4. History of other malignancies within the past 5 years, except for adequately treated carcinoma in situ of the cervix or non-melanoma skin cancer;
5. Uncontrolled primary brain tumors or central nervous system metastases, or presence of significant intracranial hypertension or neuropsychiatric symptoms;
6. Presence of severe or uncontrolled comorbidities, including but not limited to:Severe cardiac disease that remains unstable despite treatment, including myocardial infarction, congestive heart failure, unstable angina, symptomatic pericardial effusion, or unstable arrhythmia within 6 months prior to enrollment; Clearly diagnosed neurological or psychiatric disorders, including dementia or seizure disorders;Severe or uncontrolled infections;Active disseminated intravascular coagulation (DIC) or significant bleeding tendency;Significant impairment of major organ function;
7. Any other condition that, in the opinion of the investigator, would make the patient unsuitable for participation in this study.