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The aim of this study was to investigate the optimal threshold for the functional lung (FL) definition of single-photon emission computed tomography (SPECT) lung perfusion imaging.Forty consecutive stage III non-small-cell lung cancer patients underwent SPECT lung perfusion scans and PET/CT scans for treatment planning, and the images were coregistered. Total lung and perfusion lung volumes corresponding to 10, 20, …, 60% of the maximum SPECT count were segmented automatically. The SPECT-weighted mean lung dose (SWMDx%) and the percentage of FL volume receiving more than 20 Gy (Fx%V20) of different thresholds were investigated using SPECT-weighted dose–volume histograms. Receiver–operator characteristic curves were used to identify SWMD and FV20 of different thresholds in predicting the incidence of radiation pneumonitis (RP).Eleven (27.5%) patients developed RP (grades 1, 2, 3, and 4 were 10.0, 7.5, 7.5, and 2.5%, respectively) after treatment. The largest area under the receiver–operator characteristic curve was 0.881 for the ability of SWMD to predict RP with 20% as the threshold and 0.928 for the ability of FV20 with 20% as the threshold.The SWMD20% and FV20 of FL using 20% of the maximum SPECT count as the threshold may be better predictors for the risk of RP.