Material and Method: Between January 2007 and December 2014, 38 18F-FDG PET/CT examinations in 38 DTC patients with elevated sTg levels (10 ng/ml or greater, n = 33) or elevated TgAb levels (40.0 IU/ml or greater, n = 5) but negative post-therapeutic 131I WBS or diagnostic 131I WBS were included. All patients underwent total thyroidectomy and radioiodine ablation. The 18F-FDG PET/CT findings were compared with histologic (n = 16) or clinical follow-up results based on serum Tg/TgAb levels or other imaging modalities. Duration of follow-up ranged from six months to seven years (median duration = 30 months).

Results: Thirty-two studies showed true positive lesions of local recurrence (n = 23, 60.5%), distant metastases (n = 2, 5.3%) and both local recurrence and distant metastases (n = 7, 18.4%). One study (2.6%) was considered as false positive lesions at cervical nodes, which was confirmed by histological result. Four studies (10.5%) were confirmed true negative results. A patient with no abnormal 18F-FDG avidity in subcentimeter cervical lymph nodes was clarified as false negative, as subsequent follow-up results showed slow rising of Tg levels with suggestion of cervical node metastases demonstrated by neck ultrasonography. Sensitivity, specificity, and accuracy of 18F-FDG PET/CT in the present study were 96.9%, 80%, and 94.7%, respectively. Additional information of previously undiagnosed lesions was found in seven patients i.e., meningioma and metastatic lesions in brain, bone, liver, muscle, and soft tissue.

Conclusion: The 18F-FDG PET/CT is useful for detection and localization of recurrence or metastases in DTC patients with negative 131I WBS but elevated sTg or TgAb levels. This technique also provides useful additional findings for appropriate management of these patients.

Keywords: Thyroid carcinoma, 18F-FDG PET/CT, Thyroglobulin, 131I whole body scan