ctDNA-guided Selection of Adjuvant Chemotherapy Regimens for Elderly Colon Cancer Patients After Surgery: A Single-center, Randomized, Controlled Study

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.