Radical RADiotherapy and Immunotherapy for Metastatic CAncer of the Lung (RRADICAL)

Inclusion Criteria: 1. Has stage IV non-small cell lung cancer, or initially stage I-III disease with distant metastatic recurrence 2. Age ≥ 18 3. Has been receiving anti-PD-1 or anti-PD-L1 immunotherapy for at least four weeks (refer to section 4.2.1) 4. Has had restaging imaging after initiation of immunotherapy, at least 4 weeks after pre-immunotherapy baseline imaging. CT or PET/CT of at least chest/upper abdomen must be performed within 4 weeks prior to registration. For patients with history of brain metastases, brain MRI or CT is required within 4 weeks of registration; for other patients brain MRI or CT is required within 12 weeks of registration. Diagnostic PET/CT performed as part of radiation simulation can be used as the restaging imaging. 5. Most recent imaging shows measurable disease as defined by RECIST 1.1 6. Evaluation by a Stanford medical oncologist must show: 1. The patient is expected to continue on immunotherapy for at least three more months 2. Imaging must show response, stable disease, or modest progression 3. If there is modest progression, the patient must be clinically stable in terms of performance status and overall disease-related symptoms 7. Has at least one extracranial tumor safely treatable with radical-dose radiation therapy and that has not been previously treated with radiation 8. ECOG performance status 0-2 9. Has the ability to understand and the willingness to sign a written informed consent document. Exclusion Criteria: * Untreated brain metastases, if not planned to be treated in this course of radiation therapy * Pregnancy or women of childbearing potential not willing/able to use contraception during protocol treatment