Inclusion Criteria:  Sign written informed consent; With extensive small cell lung cancer; Previously received first-line standard chemotherapy, with treatment response of CR or PR; Can tolerate the radiotherapy process; Be over 18 years old Life expectancy ≥ 12 weeks; With the Eastern Cancer Cooperative Group (ECOG) score 0-1 After the systemic treatment was received, there was no brain metastasis on MR before brain preventive irradiation.  Exclusion Criteria:  Exclude subjects with central nervous system (CNS) metastasis at the first diagnosis; Pregnancy or breastfeeding; Any other conclusive medical, psychiatric and/or social reasons determined by the researcher; Subjects who have previously suffered from other malignant tumors (excluding non-melanoma skin cancer and the following carcinoma in situ: bladder, stomach, colon, endometrial, cervical/dysplasia, melanoma or breast cancer) are not allowed to participate in the study. Unless he/she has been in complete remission at least 2 years before enrolling in the study, and does not need to receive other treatments or does not need to receive other treatments during the study; Researchers believe that the treatment methods used in the study can cause harm or cause basic diseases whose toxicity is difficult to judge when judging adverse events.