Diagnostic Value of [F]-FDG and [ Ga]-FAPI-04 PET/MRI for Lymph Node Metastasis in Papillary Thyroid Cancer.

[PURPOSE] This study aimed to evaluate the diagnostic value of [F]-FDG PET/MRI for the diagnosis of neck lymph node metastasis (LNM) in patients with initially diagnosed papillary thyroid cancer (PTC) and to compare it with [ Ga]-FAPI-04 PET/MRI.

[METHODS] Thirty patients with PTC confirmed by thyroid fine-needle aspiration biopsy were prospectively enrolled and underwent [F]-FDG PET/MRI; of which, 6 additionally underwent [ Ga]-FAPI-04 PET/MRI within 3 days. According to surgical guidelines, the neck lymph node (LN) was divided into three macroscopic regions: central (VI) and left/right lateral neck (II-V). Images were semi-quantitatively and visually interpreted, and lesions' quantity, location, and uptake values were noted. Diagnostic performance of [F]-FDG PET/MRI versus US and MRI in N-staging of PTC patients based on regional analysis using postoperative histopathology as the gold standard. Whether the BRAF mutation or not affects metastatic LN radioactivity uptake. Exploring the relevance of dual tracer imaging of metastatic LN radioactivity uptake and its head-to-head comparison for diagnostic efficacy.

[RESULTS] A total of 48 macroscopic regions were surgically dissected. In terms of predicting LNM, the diagnostic efficacy of [F]-FDG PET/MRI for detecting LNM was higher than that of US and MRI, overall sensitivity, specificity, and accuracy were 71.1% vs. 60.5% vs. 65.8%, 90.0% vs.80.0% vs. 80.0%, and 75.0% vs. 64.6% vs. 68.8%, respectively (all P > 0.05). SUV of metastatic LNs on [ Ga]-FAPI-04 PET/MRI was positively correlated with [F]-FDG PET/MRI (r = 0.8564, 95%CI: 0.7208-0.9289; P < 0.0001). BRAF mutation had no significant effect on the [F]-FDG uptake level and TBR value in metastatic LN of PTC (SUV: 2.5 ± 2.3 vs. 2.2 ± 1.1; TBR: 2.9 ± 2.6 vs. 2.6 ± 1.4; all P > 0.05). The positive lesion detection rate of dual tracer imaging in 6 patients with PTC is consistent, and the degree of radioactivity uptake of [ Ga]-FAPI-04 was higher than that of [F]-FDG in both primary lesion and LNM (12.3 ± 5.7 vs. 6.9 ± 5.3;4.5 ± 3.7 vs. 3.4 ± 1.8; all P > 0.05).

[CONCLUSION] [⁸F]-FDG PET/MRI demonstrated marginally superior diagnostic performance for LNM detection compared to US and MRI, but all three modalities exhibited suboptimal sensitivity, particularly in the central region. Small sample populations revealed no significant differences in [ Ga]-FAPI-04 and [F]-FDG uptake levels in primary lesion and LNM of PTC, but relatively lower nonspecific uptake of [ Ga]-FAPI-04 pharyngeal lymphatic ring may have the potential to reduce diagnostic error in specific diseases.