Safety and Tolerability Evaluation of Low-dose Radiation in Combination With CS1001 in Relapsed SCLC Patients

Inclusion Criteria: 1. Patients with histologically or cytologically confirmed LS-SCLC or ES-SCLC and experienced progression since first-line standard platinum containing dual-drug chemotherapy. 2. Patients whose initial diagnosis was limited must undergo radical chest radiotherapy and the time of tumor progression is not less than 3 months from the end of radiotherapy, or cannot receive radical chest radiotherapy due to specific reasons 3. At least one extracranial measurable lesion (RECIST v1.1), and for a lesion that has received radiotherapy, progression of the lesion after radiotherapy must be confirmed. 4. Patients with brain metastases are allowed to receive previous radiotherapy and their condition is stable, but the time to the end of radiotherapy must not be less than 3 months. 5. No radiotherapy contraindications were judged by the radiologist 6. ECOG performance status of 0 or 1. 7. Patients with life expectancy ≥ 3 months. 8. Patients must have adequate organ function. 9. Fertile men and women of childbearing potential must agree to use an effective method of birth control from providing signed consent and for 6 months after last study drug administration. Exclusion Criteria: 1. Subjects known to have primary CNS tumors or meningeal metastases or unstable CNS metastases. 2. Patients with active autoimmune diseases or history of autoimmune diseases should be excluded. 3. Patients who have received immune checkpoint proteins/antibody/medicine (including PD-1, PD-L1, etc) for treatment. 4. Known history of HIV infection. 5. Subjects with active chronic hepatitis B or active hepatitis C . 6. Patients who have serious hypersensitive reaction to monoclonal antibodies, and have history of uncontrolled allergic asthma. 7. Known history of alcoholism or drugs abuse. 8. Subjects with history of radiation pneumonitis of grade 3 or above, regardless of recovered or not.