This randomized phase I/II trial studies the side effects and best dose of pembrolizumab when given together with stereotactic body radiation therapy or non-stereotactic wide-field radiation therapy (conventional radiation therapy) and to see how well they work in treating patients with non-small cell lung cancer. Immunotherapy with monoclonal antibodies, such as pembrolizumab, may help the body's immune system attack the cancer, and may interfere with the ability of tumor cells to grow and spread. Radiation therapy uses high-energy x-rays to kill tumor cells and shrink tumors. Stereotactic body radiation therapy uses special equipment to position a patient and deliver radiation to tumors with high precision. This method can kill tumor cells with fewer doses over a shorter period and cause less damage to normal tissue. Giving pembrolizumab together with radiation therapy may kill more tumor cells.