Multi-center observational clinical study to evaluate the application value of ctDNA methylation monitoring in efficacy assessment and relapse prediction in patients diagnosed with recurrence or metastatic colorectal cancer receiving radiotherapy plus SOC with or without PD-1.

This study will enroll patients with histologically confirmed, recurrent or metastatic colorectal cancer who are eligible to receive radiotherapy in combination with standard-of-care chemotherapy, with or without PD-1 inhibitors. The study will employ ctDNA methylation detection technology to quantitatively assess ctDNA methylation levels in patient plasma samples. Blood samples will be collected at baseline and at multiple predefined timepoints during treatment. In addition, participants will undergo standard imaging surveillance (CT/MRI) every three months. The study aims to correlate ctDNA dynamics with therapeutic response, time to recurrence, and survival outcomes. Ultimately, it seeks to evaluate the potential of ctDNA monitoring for predicting treatment efficacy and prognosis, and to explore its utility in guiding clinical management for metastatic colorectal cancer.

Inclusion Criteria:

1. Age ≥ 18 years old, regardless of gender;
2. Histologically confirmed, recurrent or metastatic colorectal cancer;
3. Eligible to receive radiotherapy in combination with standard-of-care chemotherapy, with or without PD-1 inhibitors.
4. With expected survival of more than 6 months;
5. The subjects (or their legal representative / Guardian) must sign the informed consent form, indicating that they understand the purpose of the study, understand the necessary procedures of the study, and are willing to participate in the study.

Exclusion Criteria:

1. Blood transfusion performed during operation or within 2 weeks before operation;
2. Pregnant or lactating women who have fertility and do not take adequate contraceptive measures;
3. Have a history of other malignant tumors within 5 years, except cured cervical carcinoma in situ or non melanoma skin cancer;
4. Primary brain tumor or central nerve metastasis is not under control, with obvious intracranial hypertension or neuropsychiatric symptoms;
5. Patients with the following serious or uncontrollable diseases: severe heart disease, the condition is still unstable after treatment, including myocardial infarction, congestive heart failure, unstable angina pectoris, pericardial effusion with obvious symptoms or unstable arrhythmia within 6 months before enrollment; definite neuropathy or psychosis, including dementia or seizures; severe or uncontrolled infection; active disseminated intravascular coagulation and obvious bleeding tendency;
6. Significant impairment of important organ function;
7. Other conditions in which the investigator believes that the patient should not participate in this trial.

Histologically confirmed, recurrent or metastatic colorectal cancer who are eligible to receive radiotherapy in combination with standard-of-care chemotherapy, with or without PD-1 inhibitors.