Incremental diagnostic value of [(18)F]tetrafluoroborate PET-CT compared to [(131)I]iodine scintigraphy in recurrent differentiated thyroid cancer

INTRODUCTION: Efficient therapy of recurrent differentiated thyroid cancer (DTC) is dependent on precise molecular imaging techniques targeting the human sodium iodide symporter (hNIS), which is a marker both of thyroid and DTC cells. Various iodine isotopes have been utilized for detecting DTC; however, these come with unfavorable radiation exposure and image quality ([(131)I]iodine) or limited availability ([(124)I]iodine). In contrast, [(18)F]tetrafluoroborate (TFB) is a novel radiolabeled PET substrate of hNIS, results in PET images with high-quality and low radiation doses, and should therefore be suited for imaging of DTC. The aim of the present study was to compare the diagnostic performance of [(18)F]TFB-PET to the clinical reference standard [(131)I]iodine scintigraphy in patients with recurrent DTC. METHODS: Twenty-five patients with recurrent DTC were included in this retrospective analysis. All patients underwent [(18)F]TFB-PET combined with either CT or MRI due to newly discovered elevated TG levels, antiTG levels, sonographically suspicious cervical lymph nodes, or combinations of these findings. Correlative [(131)I]iodine whole-body scintigraphy (dxWBS) including SPECT-CT was present for all patients; correlative [(18)F]FDG-PET-CT was present for 21 patients. Histological verification of [(18)F]TFB positive findings was available in 4 patients. RESULTS: [(18)F]TFB-PET detected local recurrence or metastases of DTC in significantly more patients than conventional [(131)I]iodine dxWBS and SPECT-CT (13/25 = 52% vs. 3/25 = 12%, p = 0.002). The diagnosis of 6 patients with cervical lymph node metastases that showed mildly increased FDG metabolism but negative [(131)I]iodine scintigraphy was changed: [(18)F]TFB-PET revealed hNIS expression in the metastases, which were therefore reclassified as only partly de-differentiated (histological confirmation present in two patients). Highest sensitivity for detecting recurrent DTC had the combination of [(18)F]TFB-PET-CT/MRI with [(18)F]FDG-PET-CT (64%). CONCLUSION: In the present cohort, [(18)F]TFB-PET shows higher sensitivity and accuracy than [(131)I]iodine WBS and SPECT-CT in detecting recurrent DTC. The combination of [(18)F]TFB-PET with [(18)F]FDG-PET-CT seems a reasonable strategy to characterize DTC tumor manifestations with respect to their differentiation and thereby also individually plan and monitor treatment. Future prospective studies evaluating the potential of [(18)F]TFB-PET in recurrent DTC are warranted.