Abstract: Objective To compare the differences of volume and DVH between the CT-defined and the MRI-defined parotids for intensity modulation radiated therapy(IMRT) planning on 20 patients with nasopharyngeal carcinoma(NPC).Methods Twenty patients diagnosed NPC from May 2008 to November 2008 were enrolled in this study.They received MRI and CT scan one week before radiotherapy and the images were transmitted to the treatment planning system,where the volume of parotid based on CT or MRI were contoured respectively.After fusing the CT/MRI image by using mutual information method,the difference of the volume between CT-defined and the MRI-defined parotid was evaluated.The IMRT plan was based on CT image, then the difference of DVH between CT-defined and MRI-defined parotid was compared.Results The mean volumes of parotid contoured on CT and on MRI were(24.53±1.96×7.23) cm~3 and(31.75±1.96×12.05) cm~3 respectively,the mean V_(30-CT) and V_(30-MR) were(39.29%±1.96×9.30)%and(47.73%±1.96×9.54)%respectively,and the mean D_(50-CT) and D_(50MR) were(2 716±1.96×220)cGy and(3 000±1.96×339)cGy respectively.Conclusions The contoured volume based on MRI is more accurate than that based on CT.The main difference between them is at the deep leaf of the parotid,parotid duct and subparotid.Contouring the parotid based on CT image would miss some part of the parotid and would influence the accuracy of the V30 and D50 in some cases.

Key words: Parotid, MRI, CT, IMRT, Nasopharyngeal carcinoma