Study of Low-Dose Radiotherapy (LDRT) Concurrent Cisplatin/Carboplatin Plus Etoposide With Atezolizumab for Patients With Extensive-Stage Small Cell Lung Cancer

Atezolizumab will be administered by intravenous infusion at a fixed dose of 1200 mg on Day 1 of each 21-day cycle until unacceptable toxicity or loss of clinical benefit as determined by the investigator after an integrated assessment of radiographic and biochemical data, and clinical status.

Cisplatin will be administered as intravenous infusion at a dose of 75 mg per meter squared (75 mg/m\^2) after completion of atezolizumab on Day 1 of each 21-day cycle during the induction phase (Cycles 1-4).

Carboplatin will be administered as intravenous infusion at a dose of area under the concentration-time curve (AUC) of 5 mg/mL/min on Day 1 of each 21-day cycle during the induction phase (Cycles 1-4).

Etoposide will be administered intravenously at a dose of 100 mg/m\^2 on Days 1, 2 and 3 of each 21-day cycle during the induction phase (Cycles 1-4).

Participants will receive concurrent thoracic radiation therapy (TRT) treatment, in once daily fractions, 3 Gy per fraction, to a target dose of 15 Gy in 5 fractions from Day 1-Day 5 in the first cycle.