Abstract: Objective:To improve the positioning accuracy of radiotherapy by improving the phantom fixation in radiotherapy of thoracic cancer.Methods:Twenty casess with thoracic cancer were randomly divided into conventional fixation group(without window) and mdoifed phantom fixation group(window),the setup accuracy of marked method of two groups were compared.Results:The average position error of Y-axis and X-axis of mdoifed phantom fixation group was significantly less than that of conventional fixation group from 1 to 4 weeks(P0.01) and the third week of radiotherapy respectively(P0.05-P0.01),and the data of Z-axis had no statistically significant difference(P0.05).Conclusions:The modified phantom fixation method can significantly improve the positioning accuracy of radiotherapy,which can provide the basis of rational use of phantom.