Association of Thoraco-mediastinal Radiotherapy With Maintenance Immunotherapy Treatment With Atezolizumab

This study is testing whether adding chest/mediastinal radiation therapy after chemotherapy combined with immunotherapy (atezolizumab) extends survival and prevents cancer recurrence in people with extensive-stage small cell lung cancer (a fast-growing lung cancer that has already spread) who responded to their initial treatment. **You may be eligible if...** - You are 18 or older with a confirmed diagnosis of small cell lung cancer (extensive/widespread disease) - You achieved at least a partial response after first-line chemo-immunotherapy - Your general health is adequate (ECOG 0-2) - You have had appropriate staging scans (CT, PET, brain MRI) **You may NOT be eligible if...** - You have previously received radiotherapy to the chest or mediastinum - Your cancer progressed during or after chemo-immunotherapy Talk to your doctor to see if this trial is right for you.