Abstract: Objective To analyze changes in target tumor volume during intensity-modulated radiotherapy（IMRT）for non-small cell lung cancer（NSCLC）. Methods A total of 25 NSCLC patients underwent IMRT.With the following dose series（Gy）:20，30，40，50.5 and 61，simulation computed tomography scans were carried out before each treatment to measure changes in gross tumor volume（GTV）and gross target volume of mediastinal nodes（GTVnd）. Results Both GTV and GTVnd decreased over the course of IMRT.The lowest dose causing significant changes from baseline was 40 Gy：GTV，t=5.019，P=0.000；GTVnd，t=5.267，P=0.000.IMRT was associated with significant decreases in GTV and GTVnd even after patients were grouped according to initial tumor volumes：GTV≥101.45 cm³，t=6.019，P=0.000；GTV＜101.45 cm³，t=6.491，P=0.000；GTVnd≥50.35cm³，t=5.449，P=0.000；GTVnd＜50.35 cm³ ,t=7.919，P=0.000.However，volume changes were greater for larger tumors than for smaller ones. Conclusions GTV and GTVnd gradually decreased over the course of IMRT，with larger tumors showing greater decreases. Significant decreases in GTV and GTVnd occurred even after treatment at 40 Gy.

Key words: Lung neoplasms, Non-small cell lung cancer, Intensity-modulated radiotherapy, Tumor volume, Change