At present, prophylactic cranial irradiation (PCI) is part of standard care for patients with limited-stage small cell lung cancer (SCLC) who have achieved good response after definitive thoracic radiotherapy and chemotherapy. However, the value of PCI is being challenged in the era when MRI examination of brain has been popularized.

The goal of this clinical study is to compare PCI and regular brain MRI follow-up (control arm) and regular brain MRI follow-up alone (study arm) in patients with limited-stage SCLC who have received definitive radiotherapy and chemotherapy and acheived complete remission (CR) of tumor.

The main questions to answer are:

1. Whether the 2-year brain metastasis-free survival rate of the study group is not inferior to that of the control group.
2. The difference of 2-year overall survival rate between the control group and the study group.
3. Whether the patients in the study group have better overall quality of life than those in the control group.

Participants will randomly receive either PCI and regular brain MRI follow-up or regular brain MRI follow-up alone.

Inclusion Criteria:

• Histologically/cytologically proven diagnosis of SCLC.
• Age ≥ 18 years.
• Karnofsky performance status ( KPS) ≥80.
• The limited-stage is defined as the tumor confined to one side of the chest, including ipsilateral hilar, bilateral mediastinum, and bilateral supraclavicular lymph nodes. (The definition of metastatic lymph nodes is that the short diameter ≥ 1cm or PET-CT shows increased metabolism with SUV≥2.5, or proved by mediastinoscopy/EBUS/TBNA biopsy. The thickness of pleural effusion on chest CT is less than 1cm (unless cytology proves to be malignant pleural effusion). According to 8th AJCC/UICC TNM staging system, it is the I-IIIC without intrapulmonary metastasis.
• Patients who have received definitive chest radiotherapy and chemotherapy and achieved complete remission of tumor within 4-6 weeks after the end of radio-chemotherapy (in accordance with the Response Evaluation Criteria in Solid Tumors v.1.1 including enhanced CT scan of chest and abdomen, enhanced brain MRI, bone scan and tumor markers).
• Good follow-up compliance;
• Fully understand this study, and voluntarily sign the informed consent form.

Exclusion Criteria:

• Patients with a history of malignant tumors (past or concurrent) within 5 years, excluding papillary thyroid cancer, non-malignant melanoma skin cancer and cervical carcinoma in situ.
• Patients who have received radical surgery (excluding biopsy).
• Patients with psychiatric history, pregnancy and lactation.
• Uncontrolled diabetes, hypertension, severe active infection.
• Patients with chronic diseases of central nervous system.
• Patients with contraindications of brain MRI examination.
• Other situations deemed unsuitable by the doctor in charge.