Diagnostic Performance of [18F]TFB PET/CT Compared with Therapeutic Activity [131I]Iodine SPECT/CT and [18F]FDG PET/CT in Recurrent Differentiated Thyroid Carcinoma

[¹⁸F]tetrafluoroborate ([¹⁸F]TFB) is an emerging PET tracer with excellent properties for human sodium iodide symporter (NIS)–based imaging in patients with differentiated thyroid cancer (DTC). The aim of this study was to compare [¹⁸F]TFB PET with high-activity posttherapeutic [¹³¹I]iodine whole-body scintigraphy and SPECT/CT in recurrent DTC and with [¹⁸F]FDG PET/CT in suspected dedifferentiation. Methods: Twenty-six patients treated with high-activity radioactive [¹³¹I]iodine therapy (range, 5.00–10.23 GBq) between May 2020 and November 2022 were retrospectively included. Thyroid-stimulating hormone was stimulated by 2 injections of recombinant thyroid-stimulating hormone (0.9 mg) 48 and 24 h before therapy. Before treatment, all patients underwent [¹⁸F]TFB PET/CT 40 min after injection of a median of 321 MBq of [¹⁸F]TFB. To study tracer kinetics in DTC lesions, 23 patients received an additional scan at 90 min. [¹³¹I]iodine therapeutic whole-body scintigraphy and SPECT/CT were performed at a median of 3.8 d after treatment. Twenty-five patients underwent additional [¹⁸F]FDG PET. Two experienced nuclear medicine physicians evaluated all imaging modalities in consensus. Results: A total of 62 suspected lesions were identified; of these, 30 lesions were [¹³¹I]iodine positive, 32 lesions were [¹⁸F]TFB positive, and 52 were [¹⁸F]FDG positive. Three of the 30 [¹³¹I]iodine-positive lesions were retrospectively rated as false-positive iodide uptake. Tumor-to-background ratio measurements at the 40- and 90-min time points were closely correlated (e.g., for the tumor-to-background ratio for muscle, the Pearson correlation coefficient was 0.91; P < 0.001; n = 49). We found a significant negative correlation between [¹⁸F]TFB uptake and [¹⁸F]FDG uptake as a potential marker for dedifferentiation (Pearson correlation coefficient, −0.26; P = 0.041; n = 62). Conclusion: Pretherapeutic [¹⁸F]TFB PET/CT may help to predict the positivity of recurrent DTC lesions on [¹³¹I]iodine scans. Therefore, it may help in the selection of patients for [¹³¹I]iodine therapy. Future prospective trials for iodine therapy guidance are warranted. Lesion [¹⁸F]TFB uptake seems to be inversely correlated with [¹⁸F]FDG uptake and therefore might serve as a dedifferentiation marker in DTC.