Abstract: Objective When patients are in radiotherapy,setup errors are unavoidable which may affect the precision for radiotherapy. This study was to analyze the setup errors in radiotherapy on esophageal cancer.Methods Translational errors(X,Y, and Z) and rotational errors(U,V and W) were obtained by using cone-beam computed tomography(CBCT) in radiotherapy. Patients received a total of 348 CBCT scans after initial setup,re-positioning and radiation delivery.The CBCT images were registered to the planning CT images,and then the setup errors on X,Y,Z,U,V and W degrees were analyzed.Results Setup errors were normal distribution.The systemic±random errors on X,Y,Z,U,V and W axes were(0.85±3.56 ) mm,(1.82±4.00)mm,(-2.31±2.10)mm,(0.59±0.85)°,(0.29±1.30)°and(0.40±0.86)°,respectively.Comparing the initial setup errors,the re-positioning setup errors decreased significantly on all the six axes(P<0.05).Conclusions The setup errors in Y and Z degree are more obviously,while some biggish setup errors exist in rotational errors.Measurement and correction of setup errors before each fraction by using CBCT could help to improve the precision for radiotherapy.

Key words: Image guided radiotherapy(IGRT), Cone-beam computed tomography(CBCT), Setup errors, Esophageal cancer