PURPOSE: To study whether radiation induces differential changes in plasma proteomics in patients with and without radiation-induced lung toxicity (RILT) of Grade >/=2 (RILT2). METHODS AND MATERIALS: A total of 57 patients with NSCLC received radiation therapy (RT) were eligible. Twenty patients, 6 with RILT2 with tumor stage matched to 14 without RILT2, were enrolled for this analysis. Platelet-poor plasma was obtained before RT, at 2, 4, 6 weeks during RT, and 1 and 3 months after RT. Plasma proteomes were compared using a multiplexed quantitative proteomics approach involving ExacTag labeling, reverse-phase high-performance liquid chromatography and nano-LC electrospray tandem mass spectrometry. Variance components models were used to identify the differential protein expression between patients with and without RILT2. RESULTS: More than 100 proteins were identified and quantified. After excluding proteins for which there were not at least 2 subjects with data for at least two time points, 76 proteins remained for this preliminary analysis. C4b-binding protein alpha chain, Complement C3, and Vitronectin had significantly higher expression levels in patients with RILT2 compared with patients without RILT2, based on both the data sets of RT start to 3 months post-RT (p < 0.01) and RT start to the end of RT (p < 0.01). The expression ratios of patients with RILT2 vs. without RILT2 were 1.60, 1.36, 1.46, and 1.66, 1.34, 1.46, for the above three proteins, respectively. CONCLUSIONS: This proteomic approach identified new plasma protein markers for future studies on RILT prediction.