Using ctDNA Methylation to Monitor Metastatic Colorectal Cancer Treatment (PROMET)

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.