Head-to-head comparison of 18F-FDG versus 18F-FAPI-74 PET in radioactive-iodine refractory differentiated thyroid cancer patients.

OBJECTIVE

Fibroblast activation protein inhibitor (FAPI) positron emission tomography/computed tomography (PET/CT) was reported to outperform fluorine-18-fluorodeoxyglucose (18F-FDG) PET/CT in thyroid cancer in heterogenous patient collectives. This interim analysis of a prospective study aims to determine whether 18F-FAPI-74 is superior to 18F-FDG PET/CT in the distinct subgroup of radioactive-iodine (RAI)-refractory differentiated thyroid cancer (DTC).

SUBJECTS AND

METHODS

Ten patients with recurrent DTC, detected by elevated thyroglobulin, negative RAI-scan after thyroidectomy and RAI-therapy were prospectively included. All patients underwent 18F-FAPI-74 and 18F-FDG PET/CT. The diagnostic performances were compared on a per-patient and per-lesion basis.

RESULTS

Fluorine-18-FAPI-74 PET/CT missed tumour recurrence in one patient. Fluorine-18-FDG PET/CT revealed more metastases in three patients. No management change following 18F-FAPI-74 PET/CT was documented. On a per-lesion analysis, the quantitative uptake values were significantly higher for 18F-FDG than 18F-FAPI-74 in all metastases (18F-FDG: median tumor to background ratio-TBRmax: 3.05 (1.02-9.55), 18F-FAPI-74: median TBRmax: 1.09 (0.38-5.09); P<0.001) (18F-FDG: median TBRmean: 1.71 (0.65-11.53), 18F-FAPI-74: median TBRmean: 0.64 (0.24-3.20). The detection rate was significantly higher for 18F-FDG (29/71) compared to 18F-FAPI-74 PET/CT (13/71, P<0.001).

CONCLUSION

Fluorine-18-FDG-PET/CT outperforms 18F-FAPI-74 PET/CT in RAI-refractory DTC in this prospective analysis.