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Background:Thyrotropin (TSH) stimulates thyrocyte metabolism, glucose transport, and glycolysis. The interest in using recombinant human TSH (rhTSH) stimulation of fluoro-2-deoxy-d-glucose (FDG) with positron emission tomography (PET) has been shown, but mainly for patients with high serum thyroglobulin (Tg) concentration. We evaluated the use of rhTSH-stimulated PET-FDG in patients with low serum Tg concentration.Methods:Sixty-one PET/computed tomography (CT)-FDG (Biograph Sensation 16; Siemens Medical Solutions, Knoxville, TN) were performed in 44 patients (28 women and 16 men; 51 ± 16 years) with positive Tg levels, negative or no contributive iodine-131 whole-body scintigraphy results, and no contributive morphological imaging results (ultrasound, magnetic resonance imaging, and CT). Thirty-eight patients had papillary carcinoma and six had follicular thyroid carcinoma. All patients had previously undergone total thyroidectomy and postoperative iodine ablation of thyroid bed remnant tissue. The rhTSH-stimulated PET/CT-FDG (5 MBq/kg) was performed after two 0.9 mg intramuscular doses of rhTSH (Thyrogen®; Genzyme) which were administered 48 and 24 hours before imaging, while patients continued levothyroxine (LT4). Blood sampling was performed immediately before FDG injection for measurement of serum TSH and Tg concentrations (TSH1and Tg1) and after 48 hours (TSH2and Tg2). PET/CT-FDG findings were compared with the Tg: (i) at the initial iodine treatment during T4withdrawal (Tgini), (ii) under T4(TgT4) within 3 months before the PET/CT-FDG, (iii) with Tg1, and (iv) with Tg2. PET/CT-FDG findings were correlated with the findings of histology, iodine-131 whole-body scintigraphy, morphological imaging, or clinical follow-up.Results:The mean Tginiwas 785 ± 2707 μg/L for a TSH of 73 ± 64 mU/L. The mean TgT4was 7 ± 15 μg/L (T4= 195 ± 59 μg/day; mean TSH of 0.24 ± 0.57 mU/L). Among the 44 patients, PET/CT-FDG findings were positive in 20 and negative in 24. Among the 61 PET/CT-FDG, 25 PET/CT-FDG were positive (41%). Among the 25 positive PET, the TgT4values were less than 10 μg/L for 19, including 9 true-positive patients (20% of the 44 patients). There was no difference of PET/CT-FDG results (positive vs. negative) as related to the serum Tg concentrations (p = 0.99 for Tgini, p= 0.95 for TgT4, p= 0.07 for Tg1, and p= 0.42 for Tg2). No relation was observed with PET/CT-FDG results and initial tumor size (p= 0.52) or node metastasis (p = 0.14).Conclusion:In the diagnosis of recurrent disease in patients with differentiated thyroid carcinoma and low Tg level, the sensitivity of rhTSH-stimulated PET/CT-FDG seems to be low and no correlation was observed between PET/CT-FDG findings and Tg level. However, positive PET-FDG results have been found in 9/44 (20%) patients with serum Tg levels lower than 10 μg/L. Therefore, this series shows that a cutoff value of 10 μg/L for the Tg under T4is probably not the best criteria to select patient candidates for PET/CT-FDG examination to detect the recurrence of differentiated thyroid carcinoma.16 [ABSTRACT FROM AUTHOR] |
