[(18)F]FAPI-42 PET/CT in differentiated thyroid cancer: diagnostic performance, uptake values, and comparison with 2-[(18)F]FDG PET/CT

PURPOSE: This study aimed to assess the diagnostic performance of [(18)F]FAPI-42 PET/CT and compare it with that of 2-[(18)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 [(18)F]FAPI-42 PET/CT as part of this study; of which, 11 additionally underwent 2-[(18)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 [(18)F]FAPI-42 PET/CT and biomarkers affecting the uptake of [(18)F]FAPI-42 were evaluated. In addition, the diagnostic performance and uptake of [(18)F]FAPI-42 and 2-[(18)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 [(18)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(max), 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(max): 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-[(18)F]FDG than that of [(18)F]FAPI-42 (SUV(max), 2.6 versus 2.1; P = 0.026). However, the SUV(max) of [(18)F]FAPI-42 was higher than that of 2-[(18)F]FDG in local recurrences and lymphatic lesions (SUV(max), 4.2 versus 2.9 and 3.9 versus 3.4, respectively; P > 0.05). CONCLUSION: [(18)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 [(18)F]FAPI-42 PET/CT is comparable with that of 2-[(18)F]FDG PET/CT in such patients. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00259-022-06067-2.