• Male or female aged ≥18 years.

• Documented history of histologically or cytologically confirmed ES-SCLC, prior to starting treatment with the combination of atezolizumab, carboplatin, and etoposide

• Complete Response (CR), Partial Response (PR), or Stable Disease (SD) after receiving at least three cycles, and no more than four cycles, of atezolizumab, carboplatin, and etoposide.

• Eastern Cooperative Oncology Group performance status (ECOG PS) score from 0 to 1.

• Must be ≥28 days beyond major surgical procedures such as thoracotomy, laparotomy, or joint replacement, and must not have evidence of wound dehiscence, active wound infection, or comparable major residual complications of the surgery per Investigator assessment.

• Asymptomatic brain metastases must meet ALL criteria of the following (a-d):

‣ No history of seizures in the preceding six months.

⁃ Definitive treatment must be completed ≥21 days prior to enrollment.

⁃ Must be off steroids administered because of brain metastases or related symptoms for ≥7 days.

⁃ If had previous brain irradiation, post-treatment imaging must demonstrate stability or regression of the brain metastases.

• Absolute neutrophil count (ANC) \>1500/mm3, platelet count \>100,000/mm3 within ≤28 days.

• Adequate renal function documented by serum creatinine of ≤1.5 mg/dL or calculated creatinine clearance \>50 ml/min within ≤28 days.

• Adequate hepatic function as documented by serum bilirubin \<1.5 mg/dL and aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤2.5 X upper limit of normal (ULN) within ≤28 days.

• Stable cardiac condition with a left ventricular ejection fraction ≥40% within ≤28 days.

• If female of childbearing potential (FOCBP), must have negative serum pregnancy test (serum beta-human chorionic gonadotropin \[β-hCG\]) within ≤7 days of first dose.

• FOCBP and non-sterile men who are sexually active with FOCBP must agree to use two forms of contraception including one highly effective and one effective methods beginning ≥2 weeks prior to enrollment through for four months following the last dose of study treatment.

• If male, must agree to no sperm donation during study treatment and for an additional four months following the last dose of study treatment.

• Must have voluntarily signed an informed consent in accordance with institutional policies.