This study is a prospective, single arm, phase II clinical trial. We plan to include 36 newly diagnosed ES-SCLC patients who meet the inclusion criteria and receive induction therapy (tislelizumab+EP regimen, 4-6 cycles). After completing the induction therapy, efficacy evaluation will be conducted. Patients with remission will receive tislelizumab combined with consolidation chest radiotherapy (TRT) sequentially. After the consolidation therapy is completed, they will receive tislelizumab maintenance therapy until disease progression, intolerable toxicity, or withdrawal of informed consent occurs, whichever occurs first. The treatment duration will not exceed 2 years.

Inclusion Criteria:

• Signed informed consent;
• Age 18-75 years;
• According to the judgment of the investigator, the patient was able to comply with the study protocol;
• Histologically or cytologically confirmed extensive stage small cell lung cancer (es-sclc) (according to the Veterans Administration lung cancer association [valg] staging system);
• No previous systemic treatment for ES-SCLC;
• With measurable lesions assessed by the investigator according to RECIST version 1.1;
• ECOG physical status score was 0 or 1;
• Life expectancy ≥ 3 months;
• Adequate hematology and end organ function, as defined by the following laboratory findings, which should be obtained within 14 days before the first study treatment: absolute neutrophil count (ANC) ≥ 1.5 × 10^9/l (without granulocyte colony-stimulating factor treatment); Lymphocyte count ≥ 0.5 × 10^9/l (500 / μ L); Platelet count ≥ 100 × 10^9/l (100000 / μ L); Hemoglobin ≥ 90g/l (9.0g/dl); Aspartate aminotransferase (AST), alanine aminotransferase (ALT) and alkaline phosphatase (ALP) ≤ 2.5 × upper limit of normal (ULN), with the following exceptions: Patients with confirmed liver metastasis: AST and alt ≤ 5 × ULN; Patients with confirmed liver or bone metastasis: ALP ≤ 5 × ULN; Total bilirubin ≤ 1.5 × ULN, with the following exceptions: patients known to have Gilbert syndrome: total bilirubin ≤ 3 × ULN; Creatinine clearance ≥ 60ml/min (calculated by Cockcroft Gault formula); Albumin ≥ 25g/l (2.5g/dl).

Exclusion Criteria:

• Symptomatic, untreated, or actively progressive central nervous system (CNS) metastases; ·Patients with CNS lesions who were treated and asymptomatic were eligible for this study if they met all of the following criteria: measurable lesions that met the recistv1.1 definition were outside the CNS; The patient had no history of intracranial hemorrhage or spinal cord hemorrhage; Patients did not receive stereotactic radiotherapy within 7 days before the start of study treatment, whole brain radiotherapy within 14 days before the start of study treatment, or neurosurgical resection within 28 days before the start of study treatment; Patients do not need to continue to receive corticosteroids for CNS disease. Treatment with stable doses of anticonvulsants was allowed. Metastasis was limited to the cerebellum or supratentorial region (i.e., not to the midbrain, pons, medulla oblongata, or spinal cord); There was no evidence of progression between the completion of CNS local therapy and the initiation of study therapy asymptomatic patients with CNS metastases who could be enrolled according to the assessment of other investigators could be enrolled;
• History of leptomeningeal disease;
• Poorly controlled pleural effusion, pericardial effusion or ascites requiring repeated drainage (once a month or more frequently);
• Poorly controlled or symptomatic hypercalcemia (ionized calcium >1.5mmol/l, calcium >12mg/dl or corrected calcium >uln);
• Have active or ever suffered from autoimmune disease or immunodeficiency;
• Have a history of idiopathic pulmonary fibrosis, organized pneumonia (such as bronchiolitis obliterans), drug-induced pneumonia or idiopathic pneumonia;
• Active pulmonary tuberculosis;
• Untreated chronic hepatitis B patients, chronic hepatitis B virus carriers with HBV DNA ≥ 500 iu/ml (2500 copies /ml), active hepatitis C patients: inactive HBsAg carriers, patients with stable active HBV infection after drug treatment (HBV dna<500 iu/ml (2500 copies /ml)) can be enrolled. HBV DNA testing was only performed in patients who tested positive for hepatitis B core antibody (anti HBC antibody). Patients with negative hepatitis C virus (HCV) antibody test at screening, or patients with positive HCV antibody and negative HCV RNA test at screening can be enrolled in the study. HCV RNA testing will only be performed in patients with positive hepatitis C virus (HCV) antibody. Note: Patients with detectable hepatitis B surface antigen (HBsAg) or detectable HBV DNA should be treated according to treatment guidelines. Patients treated with antiviral drugs at the time of screening should have been treated for >2 weeks before enrollment and continue to be treated for 6 months after termination of study drug treatment.