Experimental treatment

1. Drug: Tislelizumab, Carboplatin /Cisplatin, Etoposide

   • Tislelizumab (200 mg IV Q3W) in combination with chemotherapy consisting of etoposide (100 mg/m² IV Days 1-3 of each 21-day cycle) and platinum (cisplatin 75 mg/m² IV Q3W or carboplatin area under the plasma or serum concentration-time curve (AUC) 5 IV Q3W) for 4 cycles. Then maintenance consists of Tislelizumab Q3W and will continue until disease progression, loss of clinical benefit, unacceptable toxicity, or withdrawal of informed consent，up to 2 years.

2. Radiotherapy:

   • Induction therapy stage LDRT: lung lesions, 15Gy/5f;
   • Maintenance therapy phase SBRT: The main residual lesions evaluated by the investigators, 30Gy/5f；

Control group: This study refers to the Phase III RATIONALE-312 study, the reported median PFS was 4.7 months in patients treated with Tislelizumab combined with chemotherapy. This regimen has been recommened as 1L treatment for ES-SCLC in the CSCO guidelines.

Induction therapy stage LDRT: lung lesions, 15Gy/5f; Maintenance therapy phase SBRT: The main residual lesions evaluated by the investigators, 30Gy/5f；

To evaluate the efficacy of Tislelizumab + cisplatin or carboplatin + etoposide and radiotherapy in the intent to treat (ITT) Analysis Set as measured by investigator assessed progression free survival (PFS) according to Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST v1.1)

To evaluate the efficacy of Tislelizumab + cisplatin or carboplatin + etoposide +radiotherapy in the ITT Analysis Set as measured by investigator assessed overall response rate (ORR), according to RECIST v1.1