This is a Phase II, single arm, multicenter study designed to evaluate the safety and efficacy of low-dose radiotherapy (LDRT) concurrent cisplatin/carboplatin plus etoposide with atezolizumab in participants who have extensive-stage small cell lung cancer (ES-SCLC) and are chemotherapy-navïe for their extensive-stage disease.

Inclusion Criteria:

• Histologically or cytologically confirmed ES-SCLC
• No prior treatment for ES-SCLC
• Measurable disease, as defined by RECIST v1.1. Previously irradiated lesions can be considered as measurable disease only if progressive disease has been unequivocally documented at that site since radiation.
• ECOG performance status of 0 or 1
• Life expectancy >= 3 months
• Adequate hematologic and end-organ function
• For participants receiving therapeutic anticoagulation: stable anticoagulant regimen
• Negative human immunodeficiency virus (HIV) test at screening
• Negative hepatitis B surface antigen (HBsAg) test at screening
• Positive hepatitis B surface antibody (HBsAb) test at screening, or negative HBsAb at screening accompanied by either of the following: Negative total hepatitis B core antibody (HBcAb), or positive total HBcAb test followed by a negative hepatitis B virus (HBV) DNA test. The HBV DNA test must be performed for participants who have a negative HBsAg test, a negative HBsAb test, and a positive total HBcAb test.
• Negative hepatitis C virus (HCV) antibody test at screening, or positive HCV antibody test followed by a negative HCV RNA test at screening. The HCV RNA test must be performed for participants who have a positive HCV antibody test.
• For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraception
• For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use a condom, and agreement to refrain from donating sperm

Exclusion Criteria:

• Symptomatic, untreated, or actively progressing central nervous system (CNS) metastases
• History of leptomeningeal disease
• Uncontrolled tumor-related pain
• Uncontrolled pleural effusion, pericardial effusion, or ascites requiring recurrent drainage procedures
• Uncontrolled or symptomatic hypercalcemia
• Active or history of autoimmune disease or immune deficiency
• History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest computed tomography (CT) scan
• Active tuberculosis
• Significant cardiovascular disease within 3 months prior to initiation of study treatment, unstable arrhythmia, or unstable angina
• History of malignancy other than small cell lung cancer (SCLC) within 5 years prior to initiation of study treatment, with the exception of the cancer under investigation in this study and malignancies with a negligible risk of metastasis or death

Qualified researchers may request access to individual patient level data through the clinical study data request platform (www.vivli.org). Further details on Roche's criteria for eligible studies are available here ( https://vivli.org/ourmember/roche/). For further details on Roche's Global Policy on the Sharing of Clinical Information and how to request access to related clinical study documents, see here (https://www.roche.com/research\_and\_development/who\_we\_are\_how\_we\_work/clinical\_trials/our\_commitment\_to\_data\_sharing.htm).