Phase 1b/2 Study of LDRT in Combination With AK104 Plus Chemotherapy as First-line Treatment for ES-SCLC

Inclusion Criteria: * 18 to 80 years old. * Be able and willing to provide written informed consent and to comply with all requirements of study participation (including all study procedures). * Histologically or cytologically confirmed diagnosis of ES-SCLC per the Veterans Administration Lung Study Group (VALG) staging system. * No prior treatment for ES-SCLC. * Measurable disease, as defined by RECIST v1.1. * Eastern Cooperative Oncology Group performance status ≤ 1. * Life expectancy ≥ 3 months. * Adequate hematologic and end-organ function. * All female and male subjects of reproductive potential must agree to use an effective method of contraception, as determined by the Investigator, during and for 120 days after the last dose of study treatment. Exclusion Criteria: * Symptomatic or actively progressing Central nervous system metastases. * Uncontrolled carcinomatous meningitis. * Uncontrolled severe cancer pain * Uncontrolled pleural effusion, pericardial effusion or ascites requiring recurrent drainage procedures (once a month or more frequently). * Uncontrolled or symptomatic hypercalcemia. * History of autoimmune disease. * History of idiopathic pulmonary fibrosis, organizing pneumonia (such as bronchiolitis obliterans), drug-induced pneumonia, or idiopathic pneumonia, or evidence of active pneumonia on chest computed tomography (CT) during screening. * Active Tuberculosis infection. * Significant cardiovascular disease. * Major surgical procedure within 28 days prior to enrollment or anticipation of need for major surgical procedure during the course of the study. * Known additional malignancy that is progressing or requires active treatment. * Active infection requiring systemic therapy * Prior allogenic bone marrow transplantation or solid organ transplant. * Any other diseases, metabolic dysfunction, physical examination finding, or clinical laboratory finding giving reasonable suspicion of a disease or condition that contraindicates the use of an investigational drug or that may affect the interpretation of the results or render the participant at high risk for treatment complications. * Receipt of live attenuated vaccination within 4 weeks prior to the first dose of study treatment, or plan to receive live attenuated vaccine during the study. * Currently receiving the treatment of hepatitis B virus infection.. * Received approved or under development systematic anti-tumor therapy within 28 days before enrollment * Previously received immune checkpoint agonists (antibodies to CD137 targets) or immune checkpoint inhibitors (such as anti-PD-1 antibodies, anti-PD-L1 antibodies, anti-CTLA-4 antibodies, etc.) , immune cell therapy and other treatment of any immune mechanism for tumors * Treatment with systemic immunosuppressive medications within 1 week prior to enrollment.. * Known allergies or intolerant to test drugs or their excipients; or a known history of severe hypersensitivity reactions to other antibodies. * Women who are pregnant (positive pregnancy test before medication) or breastfeeding. * Received chest radiation therapy prior to the first dose.