Prophylactic Cranial Irradiation in Extensive-stage Small Cell Lung Cancer

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.