The goal of this clinical trial is to explore the disease-free survival period of elderly patients with high-risk stage II and stage III colon cancer based on ctDNA detection. The main questions it aims to answer are:

Can ctDNA detection effectively guide the assessment of disease-free survival in elderly patients with high-risk stage II and stage III colon cancer? What is the correlation between postoperative ctDNA status and patient imaging as well as prognosis in elderly patients?

Secondary objectives include:

Evaluating the correlation between postoperative ctDNA status and patient imaging, as well as prognosis, in elderly patients.

Analyzing the positive rate of postoperative ctDNA and the ctDNA clearance rate.

Additionally, an exploratory objective of this study is to investigate recurrence models for postoperative patients.

Participants will undergo ctDNA testing to assess their disease status and will be monitored for disease-free survival. Imaging studies will also be conducted to correlate with ctDNA findings. The study aims to gain a deeper understanding of the role of ctDNA in predicting prognosis and monitoring disease recurrence in elderly patients with colon cancer.

This study is a prospective, randomized controlled trial designed to explore the disease-free survival period of elderly patients with high-risk stage II and stage III colon cancer based on ctDNA detection.

1. Screening Phase Complete the routine pre-enrollment evaluation of subjects, such as medical history, pathological staging, and treatment history. Researchers will assess whether subjects meet the enrollment criteria (see inclusion and exclusion criteria). For subjects who meet the enrollment criteria, the attending physician will inform the subjects and their families, explaining the purpose, advantages, disadvantages, and the entire research process of this clinical trial. They will seek the opinions of the subjects and their families, and obtain signed informed consent from the subjects.

   Researchers need to collect baseline postoperative tissue samples for detection, which will be used for the customization of personalized probes for subsequent blood tests.

2. Adjuvant Therapy/Follow-up Phase Elderly colorectal cancer patients who meet the inclusion and exclusion criteria will undergo ctDNA-MRD testing after surgery. ctDNA-negative patients will be randomly divided into two groups in a 1:1 ratio: 1) Observation and follow-up; 2) 6-month adjuvant chemotherapy with 5-FU monotherapy. Patients who test positive for ctDNA will be randomly assigned in a 1:1 ratio to receive: 1) 6 months of 5-FU monotherapy; 2) XELOX intensive treatment group. The follow-up strategy is the same for all four groups, including but not limited to chest plain or enhanced CT, abdominal/pelvic enhanced CT, and detection of the tumor marker carcinoembryonic antigen (CEA) until disease progression occurs. All patients will undergo peripheral blood ctDNA-MRD testing again half a year after surgery.

3. Management Measures for Subjects with Disease Progression If a patient experiences disease progression, efforts should be made to continue tracking and recording the patient's subsequent anti-tumor treatment and survival outcomes, including local and/or systemic therapy.

Inclusion Criteria:

1. Patients aged 70-80, both male and female are eligible;
2. Patients with histopathologically confirmed stage II high-risk or stage III colon cancer;
3. Patients with an ECOG score of ≤2;
4. Patients who are required to undergo tissue genetic testing;
5. Subjects who voluntarily participate in this study, sign the informed consent form, have good compliance, and cooperate with follow-up visits.

Exclusion Criteria:

1. Patients with one or more severe concomitant systemic diseases that, in the investigator's opinion, would impair the patient's ability to complete the study.
2. Patients who have had a history of malignant tumors within 5 years.
3. Any unstable systemic disease (including active infection, poorly controlled diabetes, poorly controlled hypertension, unstable angina, congestive heart failure, myocardial infarction within one year, severe arrhythmia requiring medical treatment, liver, kidney, or metabolic diseases).
4. Patients suffering from severe mental illnesses.
5. Patients who have participated in other clinical trials within 30 days prior to screening.
6. Patients who are unable to undergo adjuvant chemotherapy.