Atezolizumab After Concurrent Chemo-radiotherapy Versus Chemo-radiotherapy Alone in Limited Disease Small-cell Lung Cancer

Inclusion Criteria: * Histologically or cytologically confirmed small-cell lung cancer * Previous radiotherapy to the thorax is allowed as long as the patient can receive TRT of 45 Gy. * Stage I-III according to TNM v8 ineligible for surgery provided all lesions can be included in a tolerable radiotherapy field ("limited disease") * ECOG performance status 0-2 * Measureable disease according to the RECIST 1.1 * Adequate organ function defined as: (a) Serum alanine transaminase (ALT) ≤ 2.5 x upper limit of normal (ULN); (b) Total serum bilirubin ≤ 1.5 x ULN; (c)Absolute neutrophil count (ANC) ≥ 1.5 x 10 superscr 9/L; (d) Platelets ≥ 100 x 10 superscr 9/L ; (e) Creatinine \< 100 µmol/L and calculated creatinine-clearance \> 50 ml/min. If calculated creatinine-clearance is \< 50 ml/min, an EDTA clearance should be performed * No malignant cells in pericardial or pleural fluid (at least 1 sample should be obtained if pleural fluid is present) If there is so little fluid that it cannot easily be collected, the patient is considered eligible. * Pulmonary function: FEV1 \> 1 l or \> 30 % of predicted value and DLCO \> 30 % of predicted value * Female patients of childbearing potential (Postmenarcheal, not postmenopausal (\>12 continuous months of amenorrhea with no identified cause other than menopause), and no surgical sterilization) should use highly effective contraception and take active measures to avoid pregnancy while undergoing atezolizumab treatment and for at least 5 months after the last dose. Birth control methods considered to be highly effective are listed in Appendix D of the protocol * Written informed consent Exclusion Criteria: * previous systemic therapy for SCLC or immune checkpoint blockade therapy * serious concomitant systemic disorders (for example active infection, unstable cardiovascular disease) which in the opinion of the investigator would compromise the patient's ability to complete the study, or would interfere with the evaluation of the efficacy and safety of the study treatment * lung disease requiring systemic steroids in doses of \>10 mg prednisolone (or equivalent dose of other steroid) * previous allogeneic or organ transplant * active or history of autoimmune disease or immune deficiency, including, but not limited to myasthenia gravis, myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, antiphospholipid antibody syndrome, Wegener granulomatosis, Sjögren syndrome, Guillain-Barré syndrome, or multiple sclerosis * history of idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitis obliterans), drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest computed tomography (CT) scan * live vaccine administered in the last 30 days * active infection requiring IV antibiotics * active viral hepatitis or HIV-positive * conditions - medical, social, psychological - which could prevent adequate information and follow-up * clinically active cancer other than SCLC with the exception of malignancies with a negligible risk of metastases or death (e.g. 5-years OS rate of \>90%), such as adequately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, localized prostate cancer, ductal carcinoma in situ, or stage I uterine cancer. Hormonal therapy for non-metastatic prostate or breast cancer is allowed. * pregnant or lactating women