Incremental diagnostic value of [18F]tetrafluoroborate PET-CT compared to [131I]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 ([I-131]iodine) or limited availability ([I-124]iodine). In contrast, [F-18]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 [F-18]TFB-PET to the clinical reference standard [I-131]iodine scintigraphy in patients with recurrent DTC. Methods Twenty-five patients with recurrent DTC were included in this retrospective analysis. All patients underwent [F-18]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 [I-131]iodine whole-body scintigraphy (dxWBS) including SPECT-CT was present for all patients; correlative [F-18]FDG-PET-CT was present for 21 patients. Histological verification of [F-18]TFB positive findings was available in 4 patients. Results [F-18]TFB-PET detected local recurrence or metastases of DTC in significantly more patients than conventional [I-131]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 [I-131]iodine scintigraphy was changed: [F-18]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 [F-18]TFB-PET-CT/MRI with [F-18]FDG-PET-CT (64%). Conclusion In the present cohort, [F-18]TFB-PET shows higher sensitivity and accuracy than [I-131]iodine WBS and SPECT-CT in detecting recurrent DTC. The combination of [F-18]TFB-PET with [F-18]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 [F-18]TFB-PET in recurrent DTC are warranted.