This is a single-center cross-sectional imaging study in patients with localized lung cancer undergoing immunotherapy with or without stereotactic radiation therapy as part of the companion clinical trial (NCT03217071; Pembrolizumab With and Without Radiotherapy for Non-Small Cell Lung Cancer). Each patient will undergo a single [18F]F-AraG PET exam as part of this study. [18F]F-AraG will be administered at a single time point intravenously prior to PET imaging. Whole-body PET will be acquired along with a whole body low dose CT (PET/CT) used for attenuation correction and anatomic localization of [18F]F-AraG uptake, SUV calculation, and volumetric selection for radiomic analyses.