PURPOSE: The purpose of this study was to investigate the dose-effect relation between acute esophageal toxicity (AET) and the dose-volume parameters of the esophagus after intensity modulated radiation therapy (IMRT) and concurrent chemotherapy for patients with non-small cell lung cancer (NSCLC). PATIENTS AND METHODS: One hundred thirty-nine patients with inoperable NSCLC treated with IMRT and concurrent chemotherapy were prospectively analyzed. The fractionation scheme was 66 Gy in 24 fractions. All patients received concurrently a daily dose of cisplatin (6 mg/m(2)). Maximum AET was scored according to Common Toxicity Criteria 3.0. Dose-volume parameters V5 to V70, D(mean) and D(max) of the esophagus were calculated. A logistic regression analysis was performed to analyze the dose-effect relation between these parameters and grade ≥ 2 and grade ≥ 3 AET. The outcome was compared with the clinically used esophagus V35 prediction model for grade ≥ 2 after radical 3-dimensional conformal radiation therapy (3DCRT) treatment. RESULTS: In our patient group, 9% did not experience AET, and 31% experienced grade 1 AET, 38% grade 2 AET, and 22% grade 3 AET. The incidence of grade 2 and grade 3 AET was not different from that in patients treated with CCRT using 3DCRT. The V50 turned out to be the most significant dosimetric predictor for grade ≥ 3 AET (P=.012). The derived V50 model was shown to predict grade ≥ 2 AET significantly better than the clinical V35 model (P