Purpose: To investigate the predictive value of the perfusion (Q) single-photon emission computed tomography (SPECT)-weighted dose-function histogram (DFH) obtained mid-treatment (mid-Tx) with radiotherapy (RT) for radiation-induced lung toxicity (RILT) in patients with non-small cell lung cancer (NSCLC). Materials and methods: The study population consisted of NSCLC patients who were undergoing RT treatment and enrolled in prospective imaging studies. Q-SPECT was performed prior to and during RT (at similar to 40-45 Gy). A baseline dose-volume histogram (DVH) and mid-Tx DVH based on simulation CT as well as a baseline DFH and mid-Tx DFH based on Q-SPECT were calculated. Only patients with stage III NSCLC and visible functional lung (FL) changes on the mid-Tx scan were eligible for this enriched analysis. RILT was graded according to a reported scale. Results: Forty-two stage III NSCLC patients met the criteria for inclusion. The accumulative incidence of grade >= 2 RILT was 31% in this high-risk population. Significant differences in functional metrics such as functional lung volume FV5-FV20 at increments of 5 Gy and functional MLD (FMLD) were observed between patients with and without grade >= 2 RILT (p < 0.05). Similar results were also obtained for anatomical metrics from V5-V20 and MLD (p < 0.05). The areas under the receiver operating characteristic curves (AUCs) ranged from 0.724to 0.812 for baseline DVH parameters, from 0.745 to 0.830 for midTx DVH parameters, from 0.764 to 0.878 for baseline DFH parameters, and from 0.767 to 0.891 for mid-Tx DFH parameters. Further principal components analysis showed that the AUCs were 0.814/0.817 and 0.790/0.857 for baseline/mid-Tx DVH and baseline/mid-Tx DFH, respectively. Conclusions: Mid-Tx DFH parameters based on Q-SPECT were significantly elevated in patients with grade >= 2 RILT in this study population. Among the metrics compared, mid-Tx DFH seemed to have better predictive accuracy, but this difference did not reach statistical difference. (C) 2018 Elsevier B.V. All rights reserved.