Lung cancer is a disease with high morbidity and mortality in China. The main cause of death of patients is the recurrence and metastasis of lung cancer. Blood-borne dissemination is an important way of lung cancer metastasis, and tumor cells can undergo distant metastasis through blood circulation. The entry of tumor cells into the blood circulation is one of the key steps in the development of distant metastasis, and therefore the study of circulating tumor cell detection in lung cancer patients has received increasing attention. We intend to conduct a relevant prospective clinical study in the near future to observe the role of CTC in early bloodstream metastasis screening, tumor staging, and prognostic assessment of lung cancer, and patients with advanced NSCLC (including those who progress after chemotherapy or targeted therapy), especially SCLC, are suitable to participate in the clinical study and observation. Circulating tumor cell (CTC) detection and analysis technology (CellSearch): Capable of capturing extremely minute amounts of circulating tumor cells in the blood, a single CTCs can be detected from more than 40 billion blood cells with very precise results by applying only 7.5 ml of blood sample. The basic principle is to capture and identify the minuscule number of epithelial-derived tumor cells present in the blood by immunological methods. It enables earlier detection of signs of tumor metastasis recurrence compared to traditional imaging methods such as CT.