WBRT With Hippocampal-avoidance Technique Followed by SRT for Extensive-stage SCLC With Baseline Brain Metastases

Inclusion Criteria: 1. Eastern Cooperative Oncology Group(ECOG) performance status score 0-2; 2. Small cell lung cancer confirmed by histopathology or cytology; 3. 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; 4. 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; 5. 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). 6. No history of other malignant tumors; 7. Male/female of childbearing age agreed to use contraception (surgical ligation or oral contraceptive/intrauterine device + condom) during the trial; 8. Life expectancy ≥3 months 9. Patients must be able to understand and voluntarily sign informed consent. Exclusion Criteria: 1. Patients with non-small cell lung cancer (NSCLC) components on baseline pathological examination; 2. Patients who had received any antitumor therapy prior to ES-SCLC diagnosis; 3. Patients with imaging evidence of leptomeningeal metastasis or suspected leptomeningeal metastasis with symptoms and signs; 4. patients unable to undergo contrast-enhanced MRI; 5. Patients with severe symptoms of brain metastases requiring emergency surgery to reduce intracranial pressure; 6. Patients who could not complete immobilization for radiotherapy or tolerate radiotherapy; 7. Symptomatic interstitial lung disease or active infectious/noninfectious pneumonia; 8. Patients requiring long-term corticosteroid or immunosuppressive therapy; 9. Patients who are allergic to PD-1 or PD-L1 monoclonal antibody immunotherapy or unable to receive immune maintenance therapy for other reasons; 10. Lactating or pregnant women; 11. The patient had severe autoimmune diseases: active inflammatory bowel disease (including Crohn's disease, ulcerative colitis), rheumatoid arthritis, scleroderma, systemic lupus erythematosus, autoimmune vasculitis (such as Wegener's granulomatosis), etc. 12. Medical examination or clinical findings or other uncontrollable conditions that the investigator considers may interfere with the results or increase the risk of treatment complications for the patient; 13. Patients with mental illness, substance abuse, or social problems that could affect adherence were excluded from enrollment after physician review.