Cisplatin and Etoposide Plus Radiation Followed By Nivolumab/Placebo For Locally Advanced NSCLC

Inclusion Criteria: * Pathologically (histologically or cytologically) proven diagnosis of non-small cell lung cancer (NSCLC) with unresectable, medically inoperable disease, or patients who refuse resection stage IIIA or stage IIIB disease (AJCC 7th edition) * History/physical examination within 30 days prior to registration * Computed tomography (CT) scan with IV contrast (CT scan without contrast acceptable if IV contrast is medically contraindicated) of the lung and upper abdomen through the adrenal glands within 60 days prior to registration (recommended within 30 days prior to registration) * Magnetic resonance imaging (MRI) of the brain with contrast (or CT with contrast if MRI is medically contraindicated) within 60 days prior to registration; note: the use of intravenous contrast is required for the MRI or CT (unless medically contra-indicated). * Whole-body fluorodeoxyglucose (FDG)-positron emission tomography (PET)/CT within 60 days prior to registration; Note: patients do not need to have a separate CT of chest and upper abdomen with contrast if PET/CT imaging includes a high quality CT chest with contrast. * Age ≥ 18 years * The trial is open to both genders * Zubrod Performance status of 0-1 * Forced Expiratory Volume at one second (FEV1) \> 1.2 liters; Diffusion Capacity of Lung for Carbon Monoxide (DLCO) ≥ 50% predicted * Patients must be at least 3 weeks from prior thoracotomy (if performed); if prior thoracotomy then measurable disease on imaging must be present * Negative serum pregnancy test within three days prior to registration for women of childbearing potential * An archived tumor block or punches instead block must be available for submission for programmed death-ligand 1 (PD-L1) analysis. If an archived tumor block sample cannot be shipped for this study, then two 3mm punches from the core needle biopsy blocks may be provided for analysis. Note: core or excisional biopsy is required for this study. Fine needle aspirates (FNA) and cytology specimens are not adequate for PD-L1 analysis. * Agreement of women of childbearing potential to use highly effective contraception during receipt of study drug and up to 161 days (23 weeks) from the last dose of nivolumab/placebo and men receiving nivolumab/placebo who are sexually active with women of childbearing potential to use highly effective contraception during receipt of study drug for 31 weeks from the last dose of nivolumab/placebo. Exclusion Criteria: * Definitive clinical or radiological evidence of metastatic disease * Prior or current invasive malignancy (except non-melanomatous skin cancer, localized bladder and prostate cancer) unless disease free for a minimum of 2 years (for example, carcinoma in situ of the breast, oral cavity, or cervix are all permissible) * Prior radiotherapy to the region of the study cancer that would result in overlap of radiation therapy fields. For example, patients with prior breast radiotherapy treatments would likely be excluded. * Prior systemic treatment with and anti-programmed cell death protein 1 (PD1), anti-PD-L1, anti-PD-L2, anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) (antibody, or any other antibody or drug specifically targeting T-cell costimulation or immune checkpoint pathways * Known immunosuppressive disease, for example HIV infection or history of bone marrow transplant or chronic lymphocytic leukemia (CLL) * Chronic Obstructive Pulmonary Disease (COPD) exacerbation or other respiratory illness requiring hospitalization or precluding study therapy at the time of registration. COPD requiring chronic oral steroid therapy * Unstable angina and/or congestive heart failure requiring hospitalization within the last 6 months * Transmural myocardial infarction within the last 6 months * Acute bacterial or fungal infection requiring intravenous antibiotics at the time of registration * History of symptomatic or p previously established interstitial lung disease * Positive test for hepatitis B virus surface antigen (HBV sAg) or hepatitis C virus ribonucleic acid (HCV antibody) indicating acute or chronic infection; * History of severe hypersensitivity reaction to any monoclonal antibody or allergy to study drug components * As there is potential for hepatic toxicity with nivolumab, drugs with a predisposition to hepatotoxicity should be used with caution in patients treated with nivolumab-containing regimen * Pregnancy, nursing females or women of childbearing potential and men who are sexually active and not willing/able to use medically acceptable forms of contraception; this exclusion is necessary because the treatment involved in this study may be significantly teratogenic.