Diagnostic value of preoperative 18F-FAPI PET/CT in primary papillary thyroid carcinoma and cervical lymph node metastases: a prospective study

Fibroblast activation protein (FAP)-targeted imaging using 68 Ga or 18 F has been applied in differentiated thyroid cancer (DTC), especially in postoperative follow-up, but its application value in the preoperative period remains largely unknown. This study aimed to prospectively evaluate the diagnostic value of [ 18 F] -FAPI PET/CT for primary papillary thyroid carcinoma and cervical lymph node metastasis (LNM), and explore the association between the quantitative metabolic parameters of primary lesions and LNM. In total, 43 participants (median age 50 years) with 58 thyroid lesions (45 papillary thyroid carcinoma and 13 benign lesions) were included. [ 18 F]-FAPI PET/CT and US had comparable diagnostic accuracy for primary thyroid lesions ( P >0.05). For 23 cervical regional LNM sites, 18 F-FAPI imaging outperformed US, showing substantial consistency with pathological results ( κ = 0.603, P <0.001) versus moderate consistency observed for US ( κ = 0.385, P = 0.002). The proportion of ipsilateral cervical LNM was higher in [ 18 F]-FAPI-positive thyroid lesions than negative lesions (60% vs. 20%, P = 0.035). The LNM-positive group exhibited higher FTV (FAP expression tumor volume) ( P = 0.004) and TLF (total lesion FAP expression) ( P = 0.044). LNM number correlated positively with FTV ( r = 0.474, P = 0.004) and TLF ( r = 0.398, P = 0.018). [ 18 F]-FAPI PET/CT demonstrated comparable diagnostic accuracy to US for detecting primary thyroid lesions but offered superior pathological consistency for detecting cervical LNM. [ 18 F]-FAPI -positive thyroid lesions exhibited a higher risk of ipsilateral cervical LNM. Furthermore, a higher FTV/TLF was associated with increased risk of LNM number, supporting the supplementary value of [ 18 F]-FAPI PET/CT for preoperative staging and risk stratification in DTC. Chinese Clinical Trial Registry, ChiCTR2400089382. Registered 19 March 2024, http:// www.chictr.org.cn