Objective To determine interfractional changes of tumor centroid position during stereotactic body radiation therapy (SBRT) for lung cancer using image-guided computed tomography (CT) on-rail system.Methods A total of 34 tumors were treated by SBRT using an integrated CT-LIN-AC system form octoper 2011 to May 2014.The fast helical CT scans acquired for treatment setup were retrospectively registered with simulation CT scans on the basis of the bony reference region of interest (ROI).Then,the gross target volume (GTV) was contoured on all verification CT scans and compared to the initial GTV in treatment plan.Interfractional variation of tumor centroid position was evaluated for both directional shift and vector shift.Results The mean magnitude of vector shift was (5.2 ±3.1) mm.The mean interfractional variations of tumor centroid position were (-0.69 ± 4.47) mm in anterior-posterior (AP) direction,(0.23 ±3.13) mm in superior-inferior (SI) direction and (0.36 ± 2.36)mm in right-left(RL) direction.However,interfractional variations (≥5 mm) were observed from any direction 3.3％ in RL direction,10.5％ in SI direction and 23.5％ in AP direction.Tumor volume did not decrease significantly during the SBRT treatment (P =0.078).Conclusions While the mean interfractional variations of tumor centroid position were in three directions,the vantions which were more than 5 mm took about one third,indicating additional margin for PTV were not enongh,especially in the AP direction.