Investigating Radiation-Induced Injury to Airways and Pulmonary Vasculature in Lung Stereotactic Ablative Body Radiotherapy (SAbR)

Inclusion Criteria: 1. Patients with primary or metastatic lesions in the lung, to be treated with definitive proton or photon radiotherapy. There is no limitation on the number of radiotherapy fractions, or the location/number of lesions for this study 2. Age ≥ 18 years. No gender or ethnic restrictions. 3. ECOG performance status ≤ 3 4. Ability to hold breath for \~20 seconds 5. Ability to understand and the willingness to sign a written informed consent. 6. Any types and amounts of prior therapy will be allowed for this study. 7. All men, as well as women of child-bearing potential must agree to use adequate contraception (surgical sterilization \[hysterectomy or vasectomy\], approved hormonal contraceptives \[birth control pills, patch or ring, Depo-Provera, Implanon\] or barrier method such as condom or diaphragm with spermicide or an intrauterine device) from consent until the end of study scans. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately. * A female of child-bearing potential is any woman (regardless of sexual orientation, marital status, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria: * Has not undergone a hysterectomy or bilateral oophorectomy; or * Has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months). Exclusion Criteria: 1. Children (age \<18 years). 2. Women who are pregnant, nursing, or trying to get pregnant 3. ECOG performance status \> 3