Abstract: Abstract: Objective To investigate the 18 F-FDG PET / CT features of medullary thyroid carcinoma (MTC), and to improve its diagnosis preoperatively. Methods Data of 18 F-FDG PET/CT findings and clinical materials of 7 patients with MTC confirmed by pathology were retrospectively analyzed. SUVmaxE and SUVmaxD were measured in primary tumor, and retention index (RI) was calculated. Results There were four patients with single lesion, two patients with double lesions, one patient with multiple lesions, four patients with lesions of irregular shape, four patients with lesions of unclear border, seven patients with lesions of uneven decreased density and five patients with calcifications in lesions. Uptake of 18 F-FDG in primary tumor was lower in 2 patients, their SUVmaxE values were 2.5 and 2.1 respectively, and RI values were -20%and -14.3% respectively. The other five patients showed higher metabolism in both early and delayed imaging, and SUVmaxE was 3.6-9.1 with an average of 5.8±2.1. RI was 7.5%-45.8% with the average of 21.3%±16.2%. There were five patients with cervical lymph node metastasis. The high metabolism was found in both early and delayed imaging of FDG. The ranges of SUVmaxE and SUVmaxD were 3.2-8.2 and 3.3-9.8 respectively. None of 7 patients showed distant metastasis. Conclusion 18 F-FDG PET/ CT is helpful for identifying MTC and providing information about biological behavior and evaluating metastasis of MTC.

Key words: Key words：positron-emission tomography, tomography, X-ray computed, fluorodeoxyglucose F18, diagnosis, differential, positron emissiontomography