This study is expected to enroll 182 patients with partial response or stable disease after first-line immunochemotherapy for extensive-stage small cell lung cancer and eligible for thoracic consolidation radiotherapy within 2 years. Patients were randomized 2:1 to immune single-agent maintenance therapy in combination with hyperfractionated high-dose radiotherapy and immune single-agent maintenance therapy after being assessed by the investigator as otherwise eligible for enrollment. Patients in both arms received maintenance therapy with the PD-L1 inhibitor, atezolizumab or dulvedolizumab, until disease progression, unacceptable toxicity, or loss of clinical benefit. Patients in the combined radiotherapy arm required hyperfractionated high-dose (54 Gy) radiotherapy twice daily for residual disease in the chest. Each patient will be followed for approximately 2 years.

Inclusion Criteria:

1. Fully informed written consent.
2. Age ≥ 18 years.
3. Confirmed Extensive Stage Small Cell Lung Cancer (ES-SCLC).
4. ECOG PS 0-1.
5. No previous systemic therapy except for induction immunochemotherapy for ES-SCLC.
6. Partial response or stable disease after 4-6 cycles of induction immunochemotherapy (PD-L1 inhibitor + cisplatin/carboplatin + etoposide). No more than 28 days between last tumor assessment before randomization and randomization.
7. Eligible for thoracic radiotherapy as assessed by the radiotherapy physician (The dose limits predicted for the organs at risk are as follows: bilateral lung V20 ≤ 25%, V5 ≤ 48%).
8. Patients with stable, asymptomatic CNS metastases are allowed.
9. Adequate bone marrow, renal function, and hepatic function.
10. Male or female patients of childbearing potential volunteered to use effective contraception during the study and within 6 months of the last dose of study drug.

Exclusion Criteria:

1. Prior thoracic radiotherapy.
2. History of interstitial lung disease (including but not limited to idiopathic pulmonary fibrosis), pneumonitis, drug-induced pneumonitis, idiopathic pneumonitis, or evidence of active pneumonitis on screening chest CT scan.
3. Positive testing for hepatitis B virus surface antigen (HBV sAg), hepatitis C virus ribonucleic acid (HCV RNA), or human immunodeficiency virus (HIV).
4. Leptomeningeal metastasis.
5. Uncontrolled tumor-related pain.
6. Uncontrolled pleural effusion, pericardial effusion, or ascites requiring recurrent drainage procedures (once monthly or more frequently).
7. Malignancies other than NSCLC within 5 years prior to enrollment, with the exception of those treated with expected curative outcome.
8. Active or history of autoimmune disease or immune deficiency.
9. Significant cardiovascular disease, such as New York Heart Association cardiac disease (Class II or greater), myocardial infarction, or cerebrovascular accident within 3 months prior to enrollment, unstable arrhythmias, or unstable angina.
10. Major surgical procedure other than for diagnosis within 4 weeks prior to enrollment or anticipation of need for a major surgical procedure during the course of the study.

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