Clinical impact of F-18-FDG PET in thyroid carcinoma patients with elevated thyroglobulin levels and negative I-131 Scanning results after therapy

F-18-FDG PET has been shown to effectively detect differentiated thyroid ca rcinoma (DTC) metastases with impaired iodine-trapping ability. This articl e evaluates the potential contribution of FDG PET in the follow-up of patie nts with differentiated thyroid carcinoma, elevated thyroglobulin (Tg) leve ls, and negative whole-body scan results obtained after high doses of I-131 . Methods: We prospectively assessed the ability of FDG to detect metastase s in 37 DTC patients who had undergone total thyroidectomy and radioactive ablation and presented with persistent disease, as assessed from elevated T g levels and negative results of whole-body scans performed after therapeut ic doses of I-131. Additional conventional imaging procedures were performe d to detect residual disease, and the patients were divided into 2 groups: group 1, with positive conventional imaging findings (n = 10), and group 2, with negative conventional imaging findings (n = 27). Results: FDG PET sho wed positive findings in 28 patients and accurately localized tumor sites i n 89% of them. In group 1, FDG PET confirmed 17 of 18 previously known tumo r sites and detected 11 additional sites. In group 2, FDG PET findings were positive in 19 of 27 patients with no previously detected metastases. PET was effective for both low- and high-stage tumors. The FDG data led to a ch ange in the clinical management of 29 of 37 patients with further surgical resection in 23 patients, 14 of whom achieved disease-free status, and exte rnal radiation therapy in 4 patients. Conclusion: FDG PET is able to detect metastases undetected by I-131 posttherapy whole-body scanning in patients with elevated Tg levels. It should be proposed as a first-line investigati on in patients with persistent disease but negative findings on I-131 whole -body scans after treatment.