Purpose:This study was to evaluate the setup accuracy of intensity modulated radiation therapy(IMRT)of ten patients with lung cancer by cone-beam computed tomography(CBCT)in Elekta Synergy S Linear Accelerator,to obtain and analyze the clinical setup errors of radiotherapy,and to investigate the margins of clinical target volume(CTV)based on the setup errors.Methods:The image-guided radiotherapy(IGRT)was conducted for ten patients with lung cancer chosen randomly each day,and the clinical registration between CBCT images and treatment planning CT images was carried out to keep the actual irradiation isocenter consistent with the treatment planning isocenter.The translational errors in right-left(x),superior-interferior(y),anterior-posterior(z)directions and the rotational errors in u,υ,w directions formed by x,y,z directions were calculated and analyzed for each patient in the whole course of treatment each day,and the margins of CTV were got according to M=2.5Σ+0.7δ equation on the basis of setup errors.Results:The tumors of patients and the surrounding soft tissue structures can be shown in the CBCT images more clearly,and the setup errors in translation on x,y and z axes were(0.05±0.16)cm,(0.09±0.32)cm and(-0.02±0.13)cm,respectively,while the setup errors in rotation in u,υ,w directions were(0.41±0.64)°,(-0.08±0.57)°and(-0.03±0.62)°,respectively.Based on the setup errors of patients,the margins of CTV for lung cancer were 0.82cm,1.15cm and 0.72cm,respectively.Conclusion:A certain setup errors occurred in IGRT for patients with lung cancer,and it was more obvious in superior-interferior direction.However,CBCT is capable of measuring and online correcting the setup errors,significantly reducing the setup errors of lung cancer,and enhancing the accuracy of radiotherapy,it also can evaluate the margin value and better protect the organs at risk(OARs)and normal tissues.