The Role of Tg Kinetics in Predicting 2-[ 18 F]-FDG PET/CT Results and Overall Survival in Patients Affected by Differentiated Thyroid Carcinoma with Detectable Tg and Negative 131I-Scan

Purpose The aim of this study was to assess the potential role of thyroglobulin (Tg) kinetics in predicting 2-[ 18 F]-FDG-PET/CT results and overall survival (OS) in patients affected by differentiated thyroid carcinoma (DTC) and suspected recurrence. Methods On hundred and thirty-nine patients were retrospectively included. All patients underwent 2-[ 18 F]-FDG-PET/CT due to detectable Tg levels and negative [ 131 I] whole-body scan. The last two consecutive serum Tg measurements before PET/CT were used for Tg-doubling time (TgDT) and Tg-velocity (Tg-vel) calculation. Receiver operating characteristic (ROC) curves were used to determine the optimal cutoff points for Tg, TgDT and Tg-vel for predicting PET/CT results. Results One hundred and fifteen (83%) patients had positive PET/CT for DTC recurrence, while the remaining 24 (17%) negative. Stimulated Tg before PET and Tg-vel were significantly higher in patients with a positive PET/CT scan than negative scan (average Tg 190 vs 14 ng/mL, p = 0.006; average Tg-vel 4.2 vs 1.7 ng/mL/y, p < 0.001). Instead, TgDT was significantly shorter in positive scan (average TgDT 1.4 vs 4.4 years, p < 0.001). ROC curve analysis revealed the best Tg, TgDT and Tg-vel cutoff of 18 ng/mL,1.36 years and 1.95 ng/mL/y. In patients with Tg<18 ng/mL, the PET/CT detection rate was significantly lower in patients with low Tg-vel ( p = 0.018) and with long TgDT ( p = 0.001). ATA class risk, PET/CT results and Tg before PET were confirmed to be independent prognostic variables for OS. Conclusions Tg kinetics may help to predict 2-[ 18 F]-FDG-PET/CT results in DTC patients with negative [ 131 I]WBS and detectable Tg, especially in case of low-moderate Tg.