Detection of metastases from differentiated thyroid cancer by different imaging techniques (neck ultrasound, computed tomography and [18F]-FDG positron emission tomography) in patients with negative post-therapeutic ¹³¹I whole-body scan and detectable serum thyroglobulin levels.

INTRODUCTION: DTC patients having detectable Tg and negative post-therapeutic (131)I-WBS have to be investigated by different imaging techniques to detect metastases.
PURPOSE: Comparison of neck US, CT and [18F]-FDG PET scan.
METHODS: In 49 DTC patients with biochemical disease, neck was examined by US, CT and [18F]-FDG PET. FNA was performed and Tg was determined by FNA-Tg in selected cases of suspicious lymph nodes. Thorax was examined by CT and PET. Serum Tg was measured on LT4 therapy (basal Tg) and after the stimulation with recombinant human TSH (peak Tg).
RESULTS: A thyroid remnant was seen by US, CT and PET in eight patients; recurrences were seen by US, CT and PET in six, five and five patients, respectively. Two metastatic nodes were identified by US and CT but not by PET. Lung micronodules were detected by CT in 7/49 (14.3 %) patients and by FDG PET in three of them. Basal Tg ranged from 0.5-1,725 ng/ml while peak Tg ranged from 0.5 to 2,135 ng/ml: the distribution between positive and negative patients was similar. Bone scan was negative in all cases.
CONCLUSIONS: In DTC patients with detectable Tg and negative I-131 post-therapy WBS, imaging examination revealed remnant or metastases in 43 % of cases. Remnant and recurrences were equally detected by the three techniques; US was better than [18F]-FDG PET for lymph node metastases since this latter method can give false both positive and negative results; chest examination is best made by CT versus FDG PET due to its higher spatial resolution.