This is a prospective, open,multicenter, randomized controlled phase III clinical trial. In patients with LS-SCLC who achieve remission after first-line chemoradiotherapy, the efficacy and safety of PCI or MRI surveillance is evaluated and analyzed. PCI is performed in hopes of preventing spread of cancer into the brain. The use of brain MRI alone may reduce side effects of receiving PCI and prolong patients' lifespan. MRI surveillance alone (delaying radiation until the actual brain metastasis) may be not inferior to PCI.
Histological or cytological confirmation of LS-SCLC before first-line chemoradiotherapy (according to the staging system of the Veterans' Affairs Lung Study Group (VALSG), Appendix 2)；
Remission is achieved after first-line chemoradiotherapy (CR or PR determined by RECIST v1.1)；
Brain MRI examination should be performed to exclude metastatic lesions of brain parenchyma and meninges within four weeks before enrollment；
The ECOG PS score was 0~2；
Patients with extensive SCLC (Appendix 2)；
The subjects are confirmed to have brain or meningeal metastasis before they are randomly divided into groups；
During the 5 years before the start of the study, patients with malignant tumors other than SCLC, diseases with negligible risk of metastasis or death (such as expected 5-year OS > 90%) and malignant tumors expected to be cured (such as fully treated cervical carcinoma in situ, basal or squamous cell skin cancer, localized prostate cancer treated by curable surgery, ductal carcinoma in situ treated by curable surgery)；
Previous head and neck radiation fields overlapped with PCI field；