Efficacy and Safety Study of Stereotactic Body Radiotherapy (SBRT) With or Without Pembrolizumab (MK-3475) in Adults With Unresected Stage I or II Non-Small Cell Lung Cancer (NSCLC) (MK-3475-867/KEYNOTE-867)

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 intervention

Has received an investigational agent or has used an investigational device within 4 weeks prior to the first dose of study intervention administration

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. A prior NSCLC that occurred and was treated curatively at least 2 years prior to the date of the current diagnosis would be considered a separate primary lung cancer, and therefore an additional malignancy. Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or carcinoma in situ (e.g. breast c 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, except replacement therapy

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 pembrolizumab/placebo and 180 days after the last radiotherapy dose

Have not adequately recovered from major surgery or have ongoing surgical complications

Has had an allogenic tissue/solid organ transplant

What this study is about