PURPOSE: TomoTherapy combines CT imaging with a radiation treatment delivery system.With this unique design,TomoTherapy enables efficient daily Image-guided radiation therapy(IGRT)to ensure the accuracy of the patient position before each treatment delivery.Megavoltage computed tomography(MVCT)is used in almost every TomoTherapy treatment.However,TomoTherapy beam-on times are comparable to normal irradiation times but do add about 2-3 minutes for a daily MVCT.The purpose of this study is to show that the MVCT dose can be measured in phantoms,and compare to image quality.METHODS: We studied absorbed doses measurement for Prostate cancer treatments,PTV,OAR,spine,lung,thyroid,and eye dose on phantoms(in human bone).In this measurement,we used glass dosimeter.MVCT image scanning parameter was only changed couch speed(Fine,Normal,and Coarse).This study used ROI by way of image quality.And we compared between measured dose and ROI from each MVCT image.RESULTS: The measurement doses ranged from about 28mGy to 14mGy of the PTV dose,Fine(1mm)= 28mGy,Normal(2mm)= 14mGy,Coarse(3mm)= was 14mGy.In the other organs,it was less 1mGy.The each ROI values from MVCT image are reduced by the couch speed increases.CONCLUSIONS: The measurement of the MVCT dose has shown that the fine imaging dose is approximately 28mGy per scanning.And we have present between dose to MVCT image that the negative correlation.While unnecessary radiation exposure should be avoided,but it's important to accuracy of radiation therapy.