68Ga Fibroblast Activation Protein Inhibitor PET/CT in the Detection of Metastatic Thyroid Cancer: Comparison with 18F-FDG PET/CT

Background Gallium 68 (68Ga)-labeled fibroblast activation protein inhibitor (FAPI) has been proposed as a potential radiotracer for visualizing cancerous lesions, but its utility for identifying metastatic differentiated thyroid cancer (DTC) is not well established in the literature. Purpose To evaluate the clinical utility of 68Ga-FAPI PET/CT for detecting metastatic DTC and to compare the results with those of fluorine 18 (18F) fluorodeoxyglucose (FDG) PET/CT. Materials and Methods Participants with clinically suspected or confirmed metastatic DTC were prospectively enrolled and underwent paired 68Ga-FAPI and 18F-FDG PET/CT from May to August 2020. Histopathologic results and clinical follow-up (mean, 12 months ± 0.7 [SD]; range, 11-13 months) were used as reference standards for the final diagnosis. 18F-FDG and 68Ga-FAPI uptake was compared by using the Wilcoxon signed-rank test. The McNemar test was used to compare the diagnostic accuracy of the two techniques, and the influence of various clinicopathologic characteristics on 18F-FDG and 68Ga-FAPI uptake was evaluated with Mann-Whitney and Kruskal-Wallis tests. Results In total, 35 participants (median age, 44 years; IQR, 28-58 years; 18 [51%] men) were evaluated. In all 35 participants, the 68Ga-FAPI-derived maximum standardized uptake value (SUVmax) was higher than the 18F-FDG-derived SUVmax in the metastatic lateral compartment (6.0 vs 3.5; P = .001), axillary (8.5 vs 4.3; P = .01), mediastinal lymph nodes (9.1 vs 5.0; P = .001), and pulmonary metastases (1.7 vs 1.1; P = .004). 68Ga-FAPI PET/CT had a higher sensitivity than 18F-FDG PET/CT for depicting neck lesions (83% [65 of 78; 95% CI: 73, 90] vs 65% [51 of 78; 95% CI: 54, 75], P = .01) and distant metastases (79% [87 of 110; 95% CI: 71, 86] vs 59% [65 of 110; 95% CI: 50, 68], P < .001). Conclusion Gallium 68-labeled fibroblast activation protein inhibitor PET/CT was superior to fluorine 18 fluorodeoxyglucose PET/CT for depicting metastatic differentiated thyroid cancer, especially in lymph nodes and pulmonary metastases. Published under a CC BY 4.0 license. Online supplemental material is available for this article.