A Phase 3, Randomized, Placebo-Controlled Clinical Study to Evaluate the Safety and Efficacy of Stereotactic Body Radiotherapy (SBRT) With or Without Pembrolizumab (MK-3475) in Participants With Medically Inoperable Stages I or IIA Non Small Cell Lung Cancer (NSCLC) (KEYNOTE-867)

Primary Outcome Measures  :

1. Event-free Survival (EFS) [ Time Frame: Up to approximately 6 years ]

   EFS is defined as the time from randomization to the first occurrence of any of the following events:

   • Local, regional, or distant recurrence of disease as assessed by:
     • Radiographic recurrence by blinded independent central review (BICR)
     • Positive pathology by local assessment
     • Physical examination by local assessment confirmed by positive pathology and/or radiographic recurrence by BICR OR
   • Death due to any cause. EFS will be presented.
   
   
2. Overall Survival (OS) [ Time Frame: Up to approximately 6 years ]OS is defined as the time from date of randomization to date of death from any cause. OS will be presented.
   

Inclusion Criteria:

• Has NSCLC diagnosed by histology or cytology and confirmed as Stage I or IIA NSCLC (American Joint Committee on Cancer, AJCC) by chest computed tomography (CT) and positron emission tomography (PET) scan
• Cannot undergo thoracic surgery due to existing medical illness(es) as determined by the participant's treating physician in consultation with the site's multi-disciplinary tumor board
• Has a Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2
• Is able to receive SBRT
• Has adequate organ function within 7 days prior to the start of study treatment
• A female is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies: a) not a women of childbearing potential (WOCBP) OR b) A WOCBP who agrees to use study-acceptable contraception during treatment and for at least 120 days after treatment (pembrolizumab)

Exclusion Criteria:

• Has received prior therapy with an anti-programmed cell death-1 (anti-PD-1), anti-programmed cell death-ligand 1 (anti-PD-L1), or anti PD-L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (e.g. cytotoxic T-lymphocyte-associated antigen 4 [CTLA-4], tumor necrosis factor receptor superfamily member 4 [OX-40], tumor necrosis factor receptor superfamily member 9 [CD137])
• Has received prior radiotherapy to the thorax, including radiotherapy to the esophagus, mediastinum, or breast
• Has received a live vaccine within 30 days prior to the first dose of study treatment
• 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 treatment. Note: Participants who have entered the follow-up phase of an investigational study may participate as long as it has been 4 weeks after the last dose of the previous investigational agent
• 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 treatment
• Has a known additional malignancy that is progressing or has required active treatment within the past 3 years. Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or carcinoma in situ (e.g. breast carcinoma, cervical cancer in situ) that have undergone potentially curative therapy are not excluded.
• Has a known hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients
• Has a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis
• Has a known history of Hepatitis B or known active Hepatitis C virus infection
• Has an active autoimmune disease that has required systemic treatment in past 2 years (i.e., with use of disease modifying agents, corticosteroids or immunosuppressive drugs). However, replacement therapy (e.g. thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency), while systemic, will be permitted for study eligibility.
• Has an active infection requiring systemic therapy
• Has a known history of human immunodeficiency virus (HIV) infection
• Has a known history of active tuberculosis (TB; Bacillus tuberculosis)
• Is pregnant or breastfeeding or expecting to conceive or father children within the projected duration of the study, starting with the screening visit through 120 days after the last dose of study treatment
• Has had an allogenic tissue/solid organ transplant