[18F]TFB PET/CT misses intense [124I]iodine-avid metastases after redifferentiation therapy in metastatic thyroid cancer.

Background: Fluorine 18-labelled tetrafluoroborate ([¹⁸F]TFB) is a substrate for the sodium/iodide symporter. In thyroid cancer, [¹⁸F]TFB-PET/CT may be an alternative to iodine imaging to evaluate the extent of disease, eligibility for radioiodine treatment, and success of redifferentiation therapies. We report the results of a pilot study to determine tumor uptake of [¹⁸F]TFB and compare its properties to [¹²⁴I]IodinePET/CT in patients with metastatic thyroid cancer.

Methods: Five patients were included in a prospective study. All patients received PET/CT 1 h after injection of 356 ± 12 MBq [¹⁸F]TFB and were given 230 ± 9 MBq [¹²⁴I]Iodine orally on the same day, followed by PET/CT after 48 h. Before redifferentiation therapy, patients underwent an additional baseline [¹²⁴I]Iodine PET/CT. Cases were analyzed by two board-certified specialists. Detection rates and Spearman correlation for [¹⁸F]TFB and [¹²⁴I]Iodine were calculated.

Results: Three patients had poorly differentiated thyroid cancer and received trametinib in a redifferentiation trial. Two patients had papillary thyroid cancer and did not receive redifferentiation therapy. Of the 33 lesions seen before/without redifferentiation therapy, 19 (58%) were visible on [¹⁸F]TFB and 30 (91%) on [¹²⁴I]Iodine imaging. In the patients who underwent redifferentiation therapy, 48 lesions were newly seen on [¹²⁴I]Iodine PET/CT with a median SUV_max of 3.3 (range, 0.4-285.0). All of these lesions were [¹⁸F]TFB-negative.

Conclusion: [¹⁸F]TFB failed to predict radioactive iodine uptake in patients with poorly differentiated thyroid cancer who underwent redifferentiation therapy with trametinib. It is unclear whether such discrepancies may also occur in other redifferentiation therapies or may even be encountered in redifferentiation-naïve thyroid cancer.

Trial registration number: NCT03196518, registered on June 22, 2017.