Patients with extensive-stage small-cell lung cancer (ES-SCLC) have poor prognosis, with limited treatment options. Chemo-immunotherapy is the standard 1st-line therapy for patients with ES-SCLC.When 4 cycles of etoposide+carboplatin (EC) or etoposide+cisplatin(EP) chemotherapy combined with PD-L1 inhibitor are effective, guidelines recommend additional thoracic radiotherapy.

In this study, the investigators bring radiotherapy forward, which means that after 2 cycles of EC chemotherapy plus Adebrelimab, participants with response（PR/CR/SD）will receive concurrent radiotherapy and 2 cycles of EC chemotherapy plus Adebrelimab, then maintenance therapy with Adebrelimab （Q3W）.

The purpose of this study is to explore the safety and efficacy of Adebrelimab combined with concurrent chemoradiotherapy in untreated participants with extensive-stage small cell lung cancer.

Participants receive EC chemotherapy combined with Adebrelimab for 2 cycles, and the efficacy will be evaluated. If the efficacy evaluation is SD/PR/CR, concurrent chemoradiotherapy combined with EC(2 cycles) + Adebrelimab will be initiated. After concurrent chemoradiotherapy + Adebrelimab, Adebrelimab was maintained until PD or intolerance or for at most 2 years.

Inclusion Criteria:

1. Aged 18-75 years;
2. Histologically confirmed ES-SCLC (per the Veterans Administration Lung Study Group [VALG] staging system) ;
3. Must sign a written informed consent form prior to any study specific procedures;
4. No prior treatment for ES-SCLC;
5. No more than 5 lesions (including metastases),and at least one measurable lesion that meets RECIST 1.1 evaluation criteria;
6. Life expectancy more than 3 months;
7. ECOG PS 0-1

Exclusion Criteria:

1. Previous treatment with radiochemotherapy for limited-stage small cell lung cancer;
2. Subjects who previously received systemic antitumor or Immune checkpoint inhibitor therapy ;
3. Active autoimmune disease that has required systemic treatment in the past 2 years (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive drugs); replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment;
4. Active brain metastases, Leptomeningeal metastasis(untreated central nervous system (CNS) metastases unless asymptomatic and lesion size<1cm;
5. Patients with spinal cord compression;
6. Patients with multiple liver metastases(except isolated lesion and lesion size<2cm);
7. Uncontrollable third cavity effusion (e.g. a large amount of pleural effusion, ascites, or pericardial effusion, etc.) requiring repeated drainage, which was judged by the investigator to be unsuitable for study;
8. The investigator judges that there are any patients who endanger the patient's safety, interfere with the study assessment, and have poor compliance.