• Eastern Cooperative Oncology Group(ECOG) performance status score 0-2;

• Small cell lung cancer confirmed by histopathology or cytology;

• Complete baseline imaging data (including brain enhanced MRI/CT, positron emission tomography(PET/CT) or chest enhanced CT+ bone scan + neck and abdomen B ultrasound /CT) should be obtained before first-line treatment;

• Patients with initial diagnosis of ES-SCLC with brain metastases who planned to receive at least 4 cycles of standard platinum-based doublet chemotherapy combined with immunotherapy (PD-1 or PD-L1 monoclonal antibody) as first-line treatment, and who met the organ function requirements as judged by the investigator;

• Brain metastases assessed by contrast-enhanced MRI met the criteria for SRS (less than or equal to 10 brain metastases, maximum tumor volume less than 10ml, maximum tumor diameter less than 3cm, total tumor volume less than 15ml, and no evidence of leptomeningeal metastasis).

• No history of other malignant tumors;

• Male/female of childbearing age agreed to use contraception (surgical ligation or oral contraceptive/intrauterine device + condom) during the trial;

• Life expectancy ≥3 months

• Patients must be able to understand and voluntarily sign informed consent.