Abstract: Abstract:Objective To assess the residual set-up error of the on-line kilovoltage cone-beam CT (KVCBCT) guided radiotherapy for thoracic tumor patients. Methods From Mar 2008 to Aug 2008, 21 patients with pathologically confirmed thoracic tumor were included in this study.On-line imaging-guided set-up error correction was performed weekly for each patient.Set-up error, before and after correction, was documented. Results A total of 225 kVCBCT scans were obtained during 6 weeks.The initial average set-up error(±s) was(-0.2±4.6)mm, (0.5±4.5)mm and (-0.8±4.4)mm in the left-right (LR), superior-inferior (SI) and anterior-posterior (AP) direction, respectively.After online correction, average residual set-up error (±s) was(-0.2±1.3)mm, (0.1±1.3)mm and (0.3±1.3)mm in the LR, SI and AP directions, respectively.According to the formalisms suggested by van Herk et al, 3mm can be applied to the margin of clinical target volume (CTV) to planning target volume (PTV), when using the on-line correction. Conclusion On-line KVCBCT guidance can improve the set-up accuracy of thoracic tumor patient's radiation.