The prognosis of extensive-stage small cell lung cancer is still very poor, even for those who received chemotherapy and immunotherapy. This experimental study is a real-world research design to evaluate the effectiveness and safety of prophylactic cranial irradiation in patients with extensive-stage small cell lung cancer who with first-line chemotherapy combined with immunotherapy.

This study is a prospective trial. This study included ES-SCLC patients who response after standard first-line treatment, and aimed to explore the safety and effectiveness of prophylactic cranial irradiation in this treatment modality.

The primary endpoint is progression-free survival in the brain.The secondary endpoints includes OS, PFS. The indicators for evaluating safety are the incidence and severity of adverse events.The exploratory endpoint is the molecular biomarkers for efficacy and toxicity predicting from tumor tissue and peripheral blood TMB.

The trial was designed by the National Cancer Center/Cancer Hospital of the Chinese Academy of Medical Sciences and Peking Union Medical College. The hypothesis is that preventive brain irradiation is safe and effective in the treatment of extensive SCLC combined with chemotherapy and immunotherapy.

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.

It is planned to enroll patients who have completed and response to first-line chemotherapy combined with immunotherapy, Later immunotherapy treatments are open