This trial is evaluating the survival benefit from using concurrent chemoradiation with and without targeted therapy for the treatment of small cell lung cancerA Randomized, Double-blind, Placebo-controlled Phase 3 Study of Pembrolizumab (MK-3475) in Combination With Concurrent Chemoradiation Therapy Followed by Pembrolizumab With or Without Olaparib (MK-7339), Compared to Concurrent Chemoradiation Therapy Alone in Participants With Newly Diagnosed Treatment-Naïve Limited-Stage Small Cell Lung Cancer (LS-SCLC)

Exclusion

1. Has history, current diagnosis, or features suggestive of myelodysplastic syndrome/ acute myeloid leukemia (MDS/AML).
2. Has received prior therapy with an anti-programmed cell death 1 (anti-PD-1), anti-programmed cell death ligand 1 (anti-PDL1), or anti- programmed cell death ligand 2 (anti-PD-L2) agent or with an agent directed to another stimulatory or coinhibitory T-cell receptor
3. Has received prior therapy with olaparib or with any other polyadenosine 5'diphosphoribose (polyADP ribose) polymerization (PARP) inhibitor.
4. Had major surgery <4 weeks prior to the first dose of study intervention (except for placement of vascular access).
5. Is currently participating in or has participated in a study of an investigational agent or has used an investigational device within 4 weeks prior to the first dose of study intervention.
6. Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior the first dose of study intervention.
7. Has a known additional malignancy that is progressing or has required active treatment within the past 5 years. Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, superficial bladder cancer, or carcinoma in situ (e.g., breast carcinoma, cervical cancer in situ) that have undergone potentially curative therapy are not excluded.
8. Has severe hypersensitivity (≥ Grade 3) to study intervention and/or any of its excipients.
9. Has an active autoimmune disease that has required systemic treatment in past 2 years
10. Has a history of (non-infectious) pneumonitis/interstitial lung disease that requires steroids
11. Has an active infection requiring systemic therapy.
12. Has a known history of human immunodeficiency virus (HIV) infection or Hepatitis B or known active Hepatitis C virus infection.