The diagnostic value of ⁶⁸Ga-FAPI PET/CT in patients with suspicious recurrent or metastatic differentiated thyroid...

This study aimed to compare the detection rates and uptake values of gallium-68 (⁶⁸Ga)-labeled fibroblast activation protein inhibitor (FAPI) PET/CT with those of fluorine-18 (¹⁸F)- labeled fluorodeoxyglucose (FDG) PET/CT in patients with suspicious recurrent or metastatic differentiated thyroid cancer (DTC), and to explore the association of these rates and values with diverse clinical-pathological characteristics and degrees of differentiation based on visual analysis and semi-quantitative parameters.

A retrospective cohort comprising 56 patients with elevated Tg, positive anti-Tg antibodies, or thyroid ultrasound suggesting recurrence or metastasis was enrolled. All patients had previously undergone ⁶⁸Ga-FAPI PET/CT and ¹⁸F-FDG PET/CT scans. Lesion detection and uptake values were analyzed visually and semi-quantitatively, with final diagnosis confirmed by histopathology or clinical follow-up.

¹⁸F-FDG PET/CT identified more positive patients (86% vs. 76%) and demonstrated a higher detection rate in lymph node metastases (71% vs. 59%, p=0.029). ⁶⁸Ga-FAPI showed higher uptake (SUVmax) in pulmonary metastases (3.31 vs. 1.74, p=0.017) but a lower tumor-to-background ratio (TBR) in lymph nodes (2.20 vs. 4.10, p<0.001). Among the clinical and pathological characteristics assessed, Tg levels, short axis diameter (SAD) of lymph node, and iodine uptake displayed significant differences between ⁶⁸Ga-FAPI uptake negative (FAPI-) and ⁶⁸Ga-FAPI uptake positive (FAPI+) groups (p=0.025, p<0.001, p=0.003). However, only the SAD of lymph node showed a moderate positive correlation with the SUVmax value of ⁶⁸Ga-FAPI in the lesion (r=0.393, p=0.042).

Our findings indicate that ⁶⁸Ga-FAPI PET/CT does not demonstrate significantly superior diagnostic performance when compared to ¹⁸F-FDG PET/CT in patients with recurrent or metastatic DTC. Nevertheless, for patients with enlarged metastatic lymph nodes, elevated Tg levels, and iodine-affinity lesions, ⁶⁸Ga-FAPI PET/CT could represent a valuable diagnostic tool.