In order to better understand radiation-induced lung toxicity, the investigator proposes a novel functional avoidance approach that incorporates central as well as peripheral BSS segments in the treatment planning process in order to quantify and account for their respective radiosensitivities. Specifically, the investigator proposes a systematic study that involves acquiring pre- and post-SAbR high-resolution CT and SPECT V/Q scans from lung cancer patients who receive radiotherapy, followed by auto-segmentation of the BSS elements using virtual bronchoscopy.