Comparison of 68Ga-FAPI-04 and 18F-FDG PET/CT for diagnosis of metastatic lesions in patients with recurrent papillary thyroid carcinoma.

We aimed to evaluate the gallium-68-labeled fibroblast-activation protein inhibitor (68Ga-FAPI) positron emission tomography/computed tomography (PET/CT) in localizing papillary thyroid carcinoma (PTC) foci in patients with biochemical relapse. Papillary thyroid carcinoma has achieved biochemical recovery after appropriate treatment and had biochemical relapse in the last follow-up were included in this retrospective study. Gallium-68-FAPI and fluorine-18-fluorodeoxyglucose (68F-FDG) PET/CT were performed to detect recurrence foci.Biochemically relapsed patients who underwent total thyroidectomy and were diagnosed with pathologically differentiated thyroid cancer were included in our study. Gallium-68-FAPI and 18F-FDG PET/CT imaging methods were used to determine the focus of metastasis or recurrence in all patients.Among 29 patients enrolled to the study, pathological subgroups were papillary (n=26) and poorly differentiated (n=3) PTC. Anti-thyroglobulin (TG) antibody positivity were noted in 5 of the patients, while all 29 of them were TG positive and had been consist of three groups as follows: 2-10ng/mL (n=4), 11-300ng/mL (n=14), 301ng/mL and above (n=11). Recurrence was detected in 72.4% (n=21) and 86% (n=25) of the patients via 18F-FDG and 68Ga-FAPI, respectively. Accuracy of detection noted as 100% (5/5), 75% (3/4), and 92.9% (13/14) in groups with the anti-TG antibody positivity, TG levels of 2-10ng/mL and 11-300ng/mL, respectively, when the two imaging modalities were utilized together. Furthermore, accuracy of 68Ga-FAPI was 100% (11/11) in the group with TG levels of 301ng/mL and above, whereas accuracy of 18F-FDG was 81.8% (9/11). Lastly, median maximum standardized uptake value (SUVmax) of recurrent lesions detected by the 68Ga-FAPI (median SUVmax: 6.0) were statistically higher than the ones detected by the 18F-FDG (median SUVmax: 3.7) (P=0.002).In recurrent PTC especially in case of higher TG levels, 68Ga-FAPI can be used in patients with inconclusive 18F-FDG findings.