Radiation therapy (RT) is widely used to treat lung cancer, but damage to surrounding healthy tissues can lead to compromised lung function. In this study, patients undergoing RT were imaged pre- and post-treatment using hyperpolarized 129Xe gas exchange MRI to assess for RT-induced changes in regional lung function. At 3-months post-treatment, a dose-response was evident in ventilation and gas exchange. Lung regions receiving ≥20Gy exhibited significantly increased barrier uptake and decreased RBC transfer. This may help radiation oncologists further understand the dose-dependence of RT-induced lung injury, and design dose distributions with fewer treatment toxicities.