[F]FAPI-42 PET/CT in differentiated thyroid cancer: diagnostic performance, uptake values, and comparison with 2-[F]FDG PET/CT.

Purpose: This study aimed to assess the diagnostic performance of [F]FAPI-42 PET/CT and compare it with that of 2-[F]FDG PET/CT in patients with differentiated thyroid cancer (DTC) with biochemical elevations in Tg or anti-Tg antibodies.

Methods: A total of 42 patients with DTC with biochemical elevations in Tg or anti-Tg antibodies underwent [F]FAPI-42 PET/CT as part of this study; of which, 11 additionally underwent 2-[F]FDG PET/CT within 7 days. Images were semi-quantitatively and visually interpreted, and the quantity, location, and uptake values of lesions were noted. The diagnostic capacity of [F]FAPI-42 PET/CT and biomarkers affecting the uptake of [F]FAPI-42 were evaluated. In addition, the diagnostic performance and uptake of [F]FAPI-42 and 2-[F]FDG were compared, and the correlation between lesion diameter and quantitative parameters was investigated.

Results: A total of 161 lesions were detected in 27 (64%) patients on [F]FAPI-42 PET/CT. FAPI-positive local recurrence showed the highest uptake intensity, followed by lymphatic, other site-associated (bone and pleura), and pulmonary lesions (mean SUV, 4.7 versus 3.7 versus 3.0 versus 2.2, respectively; P < 0.0001). The levels of TSH, Tg, and Tg-Ab did not affect the uptake value of lesions (median SUV: 2.4 versus 3.2, P = 0.56; 2.9 versus 2.4, P = 0.0935; 2.8 versus 2.6, P = 0.0525, respectively). A total of 90 positive lesions were detected in 7 patients using both modalities. All positive lesions showed statistically higher uptake of 2-[F]FDG than that of [F]FAPI-42 (SUV, 2.6 versus 2.1; P = 0.026). However, the SUV of [F]FAPI-42 was higher than that of 2-[F]FDG in local recurrences and lymphatic lesions (SUV, 4.2 versus 2.9 and 3.9 versus 3.4, respectively; P > 0.05).

Conclusion: [F]FAPI-42 can be used for detecting lesions and reflecting FAP expression during local recurrence and metastasis in patients with DTC with biochemical elevations in Tg or anti-Tg antibodies. The diagnostic performance of [F]FAPI-42 PET/CT is comparable with that of 2-[F]FDG PET/CT in such patients.