This study is a single arm, open, multicenter phase II study. The main purpose of this study was to evaluate preliminary efficacy and safety of Tislelizumab combined with thoracic radiotherapy as first line therapy for Extensive stage small cell lung cancer.

Inclusion Criteria:

• Patients aged between 18 to 80 years old (inclusive of 18th and 80th birthdays), male or female, at the time of signing the informed consent form;
• Confirmed small cell lung cancer of extensive stage by histology or cytology;
• At least one measurable lesion by imaging studies (according to RECIST 1.1) with a long diameter of ≥10 mm as examined by spiral CT or MRI;
• Within 3 days prior to treatment, an ECOG score of 0 to 1;
• No prior antitumor treatment for extensive stage disease (if the patient has previously received chemotherapy and/or radiotherapy in the limited stage of SCLC, the treatment intent must have been curative, and there must be at least a 6-month treatment-free interval between the end of chemotherapy, radiotherapy, or chemoradiotherapy and the diagnosis of extensive stage SCLC);
• Expected life span of ≥3 months;
• Good function of vital organs;
• The subject voluntarily joins this study, signs the informed consent form, has good compliance, and cooperates with follow-up.

Exclusion Criteria:

• Concurrent severe respiratory diseases: such as pulmonary fibrosis;
• Presence of psychiatric disorders, hematologic diseases, autoimmune diseases, and severe primary diseases of the heart, brain, liver, or kidneys;
• Uncontrolled active infections;
• Known or suspected allergies to the study medication and its excipients;
• Female patients who are pregnant or breastfeeding, or women of childbearing potential with a positive baseline pregnancy test;
• Prior use of antitumor treatment targeting the PD-(L)1 pathway.