Background: The risk factors for radiation pneumonitis (RP) in patients with chronic obstructive pulmonary disease\n(COPD) are unclear. Mean lung dose (MLD) and percentage of irradiated lung volume are common predictors of\nRP, but the most accurate dosimetric parameter has not been established. We hypothesized that the total lung\nvolume irradiated without emphysema would influence the onset of RP.\nMethods: We retrospectively evaluated 100 patients who received radiotherapy for lung cancer. RP was graded\naccording to the Common Terminology Criteria for Adverse Events (version 4.03). We quantified low attenuation\nvolume (LAV) using quantitative computed tomography analysis. The association between RP and traditional\ndosimetric parameters including MLD, volume of the lung receiving a dose of �2 Gy, � 5 Gy, � 10 Gy, � 20 Gy, and\n�30 Gy, and counterpart measurements of the lung without LAV, were analyzed by logistic regression. We\ncompared each dosimetric parameter for RP using multiple predictive performance measures including area under\nthe receiver operating characteristic curve (AUC) and integrated discrimination improvement (IDI).\nResults: Of 100 patients, RP of Grades 1, 2, 3, 4, and 5 was diagnosed in 24, 12, 13, 1, and 1 patients, respectively.\nCompared with traditional dosimetric parameters, counterpart measurements without LAV improved risk prediction\nof symptomatic RP. The ratio of the lung without LAV receiving �30 Gy to the total lung volume without LAV most\naccurately predicted symptomatic RP (AUC, 0.894; IDI, 0.064).\nConclusion: Irradiated lung volume without LAV predicted RP more accurately than traditional dosimetric parameters.
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