prophylactic cranial irradiation （PCI）was verified to decrease the brain metastases rates and improve the overall survival（OS）for patients with limited stage small cell lung cancer.We hypothesis that patients with extensive-stage small cell lung cancer after chemotherapy and thoracic radiation can also benefit from PCI.

Patients with extensive-stage small cell lung cancer achieving CR or PR after chemotherapy and thoracic radiation therapy were randomly divided into observation group and prophylactic cranial irradiation (PCI) group to evaluate the effect of PCI on survival after radiation therapy for thoracic lesions. To preliminarily analyze the clinical characteristics of patients who would benefit from thoracic radiation therapy plus PCI. To explore the optimal treatment modality after chemotherapy + thoracic radiation therapy in patients with extensive-stage SCLC.

Inclusion Criteria:

• 18-65 years old,KPS≥70
• Pathological or cytological diagnosis of stage IV small cell lung cancer (by using AJCC 7th edition staging criteria);
• 1-3 extra-cranial metastatic organs before chemotherapy; 4-6 cycles of CE or EP ；
• CR or PR after chemotherapy；
• no grade ≥II side effects;
• the estimated overall survival time ≥ 3 months;
• no serious medical disease or major organ dysfunction.

Exclusion Criteria:

• Patients with brain or nervous system metastases;
• Presence of high grade radiation comorbidities;
• Patients who are pregnant or lactating