Background:Medullary thyroid carcinoma (MTC) is a rare disease,but it exhibits more aggressive behaviors.The aim of this study was to improve the diagnostic accuracy of MTC before surgery by analyzing the clinical and ultrasonic data of patients with MTC.Methods:The study included 71 patients (96 lesions) with histopathologically proven MTC between April 2011 and September 2016 in the Cancer Hospital of Chinese Academy of Medical Sciences and Peking Union Medical College.The clinical characteristics and sonographic findings were retrospectively reviewed and compared between the ultrasonic correct diagnosis group and the ultrasonic misdiagnosis group with the t test or Mann-Whitney U test for quantitative parameters and the x2 test or Fisher exact test for qualitative parameters.Results:Compared with the ultrasonic correct diagnosis group,the proportion of the cystic change in the ultrasonic misdiagnosed group was high (25.0％ vs.4.2％),the uncircumscribed margin and irregular shape proportions were low (20.8％,58.3％ vs.74.7％,87.3％),calcification was relatively rare (20.8％ vs.56.3％),and rich vascularity was relatively rare (25.0％ vs.78.9％).Conclusions:In the case of atypical MTC,such as cystic change,circumscribed margin,regular shape,no calcification,no rich vascularity,and normal cervical lymph nodes,MTC is easily misdiagnosed as benign by ultrasound.Therefore,ultrasound,cytology and serum calcitonin should be comprehensively evaluated for a preoperative diagnosis of MTC.