After radiotherapy for small cell lung cancer, there is still a possibility of metastasis of the cancer cells, because radiotherapy usually does not kill all the cancer cells. Compared with non-small cell lung cancer, small cell lung cancer proliferates fast and metastasizes at an early stage. 90% of small cell lung cancer patients already have intrathoracic and distant metastasis at the time of diagnosis, so surgery is not recommended and radiotherapy and medication are the most common treatments nowadays. However, even if radiotherapy and medication are combined at the same time, they cannot kill all the cancer cells, but only most of them, and there is still a great possibility of metastasis in the patient, and radiotherapy cannot completely kill the metastatic cancer cells. Patients with small cell lung cancer still need to actively consult the doctor and follow the doctor’s instructions for reasonable treatment.