The clinical diagnosis and treatment of small pulmonary nodules (suspected to be lung metastases) in advanced colorectal cancer patients remain controversy. Previous studies have shown that tumor-informed circulating tumor DNA (ctDNA) blood testing can sensitively detect residual cancer. Postoperative ctDNA in colorectal cancer patients is a valuable biomarker to identify minimal residual disease (MRD) after radical resection, which is possibly useful in redefining the risk group of patients and guiding postoperative treatment. This study aimed to explore the clinical value of therapeutic strategies based on tumor-informed ctDNA test in advanved colorectal cancer patients with small pulmonary nodules.

Inclusion Criteria:

1. Age ≥ 18 and ≤80 years old, regardless of gender;
2. Pathologically confirmed as adenocarcinoma of advanced colorectal cancer;
3. Lung lesions only, which was considered as metastatic by imaging consultation, and clinically diagnosed as lung metastasis of colorectal cancer;
4. Multiple lung lesions are allowed, but the maximum lesion diameter should be less than or equal to 2 cm;
5. For lesions larger than 1 cm, local treatment is planned, such as radical surgical resection or local radiotherapy, radio frequency ablation or interventional therapy (absolute alcohol treatment or cryotherapy);
6. Eastern Cooperative Oncology Group (ECOG) score 1 ~ 2;
7. 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;
8. Surgical specimens or puncture specimens containing tumor tissue are available;
9. 20 mL of peripheral blood are available (10 mL per tube, two tubes in total);
10. Agreed to follow up for at least 2 years.

Exclusion Criteria:

1. The pathology was not confirmed by enteroscopic biopsy or biopsy of metastatic lesions;
2. Patients with stage I-III colorectal cancer;
3. Primary lung cancer, GGO, tuberculosis and other non metastatic conditions were excluded after diagnosis by imaging consultation;
4. Presence of metastasis other than lung;
5. Insufficient organ function, such as severe abnormal hemogram, abnormal liver and kidney function;
6. Any signs of severe or uncontrolled systemic diseases that the researcher believes may have a significant patient risk/benefit balance, including uncontrolled hypertension, severe infection, hepatitis B, hepatitis C and human immunodeficiency virus;
7. History of alcoholism or drug abuse;
8. Pregnant or lactating patients.