Objective To analyze setup errors and guide the calculation of margins from clinical target volume (CTV) and planning target volume (PTV) in esophageal cancer patients treated with tomothcrapy by the MVCT image-guided system.Methods Sixty-four esophageal canccr patients trcated with tomotherapy in our hospital in 2016 were randomly selected.MVCT images were acquired after patients' positioning and co-registered with KVCT images.The setup errors of x,y,and z translations and roll rotation were analyzed with the t-test or one-way ANOVA.Meanwhile,PTV margin was calculated based on the formula of M =2.5 Σ + 0.7δ Results According to the formula,the CTV-PTV margins in the x,y and z directions are slightly different between cancers located in the cervical,upper thoracic,middle thoracic,and lower thoracic segments.In patients with upper thoracic esophageal cancer,the average setnp error in the yaxis was lower when the head-neck-shoulder thermoplastic film fixation was used than when somatic thermoplastic film fixation (P=0.000);the setup errors of z-axis with somatic thermoplastic film fixation in the fifth and sixth weeks were slightly less than those in the first several weeks (P =0.036);the setup errors acquired by three image registration patterns were similar (x-axis P=0.868,y-axis P=0.491,z-axis P=0.169,roll P=0.985).Conclusions In the treatment of patients with esophageal cancer,the setup errors are large,but the MVCT in the TOMO HD system can greatly reduce the setup errors,ensuring the accuracy of each treatment.It is further recommended that in clinical practice,different CTV-PTV margins should be used for the treatments of esophageal cancers located in different segments.Patients with upper thoracic esophageal cancer are advised to use the head-neck-shoulder thermoplastic film fixation.