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Purpose: To compare hyperpolarized helium-3 margnetic resonance imaging (He-3-MRI) acquired from non-small cell lung cancer (NSCLC) patients before and after external beam radiotherapy (EBRT). Methods and materials: In a ethics Committee-approved prospective study, five patients with histologically confirmed NSCLC gave written informed consent to undergo computed tomography (CT) and He-3-MR ventilation imaging 1 week prior to and 3 months after radiotherapy. Images were registered to pre-treatment CT using anatomical landmark-based rigid registration to enable comparison. Emphysema was graded from examination of the CT. MRI-defined ventilation was calculated as the intersection of He-3-MRI and CT lung volume as a percentage of the CT lung volume for the whole lung and regions of CT-defined pneumonitis. Results: On pre-treatment images, there was a significant correlation between the degree of CT-defined emphysema and He-3-MRI whole lung ventilation (Spearman’s rho = 0.90, p = 0.04). After radiation therapy, pneumonitis was evident on CT for 3/5 patients. For these cases. He-3-MRI ventilation was significantly reduced within the regions of pneumonitis (pre: 94.1 +/- 2.2%, post: 73.7 +/- 4.7%; matched pairs Student’s t-test, p = 0.02, mean difference = 20.4%, 95% confidence interval 6.3-34.6%). Conclusions: This work demonstrates the feasibility of detecting ventilation changes between pre- and post-treatment using hyperpolarized helium-3 MRI for detecting ventilation changes between pre- and post-treatment using hyperpolarized helium-3 MRI for patients with NSCLC. Pre-treatment, the degree of emphysema and He-3-MRI ventilation were correlated. For three cases of radiation pneumonitis, He-3-MRI ventilation changes between pre- and post-treatment imaging were consistent with CT evidence of radiation-induced lung injury (C) 2010 Elsevier Ireland Ltd. All rights reserved. Randiotherapy and Oncology 97 (2010) 244-248