Clinical Utility of F-18 Labeled Fibroblast Activation Protein Inhibitor (FAPI) for Primary Staging in Lung Adenocarcinoma: a Prospective Study

Purpose The purpose of this study was to compare the primary staging of F-18 labeled fibroblast activation protein inhibitor ([ 18 F]F-FAPI) with that of F-18 labeled fluordesoxyglucose positron emission tomography/computed tomography ([ 18 F]F-FDG PET/CT) in patients with lung adenocarcinoma (LAD). Procedures We prospectively analyzed the images of LAD patients who underwent [ 18 F]F-FAPI and [ 18 F]F-FDG PET/CT between May 2020 and August 2021. [ 18 F]F-FAPI and [ 18 F]F-FDG uptakes were compared using the paired samples t test, and lesion numbers were compared using the Wilcoxon signed-rank test. Results Thirty-four LAD patients were evaluated. Patients showed high [ 18 F]F-FAPI uptake in primary lesions (SUVmax 12.54 ± 3.77). Both [ 18 F]F-FAPI and [ 18 F]F-FDG had 100% detection rates for primary tumors. However, [ 18 F]F-FAPI showed higher SUV max than [ 18 F]F-FDG in lesions of the lymph nodes, pleura, bones, and other tissues (all P ≤ 0.05). Although the absolute uptake values of [ 18 F]F-FAPI in brain lesions were lower than those of [ 18 F]F-FDG (1.56 ± 2.19 vs.7.34 ± 3.54, P < 0.0001), the tumor-to-background (T/B) ratios were significantly higher than those of [ 18 F]F-FDG (9.53 ± 12.07 vs.1.01 ± 0.49, P < 0.0001). Generally, [ 18 F]F-FAPI PET/CT could visualize more total lesions than [ 18 F]F-FDG (554 vs.464, P = 0.003), especially in lymph nodes (258 vs.229, P = 0.039), the brain (34 vs.9, P = 0.002), and pleura (56 vs.30, P = 0.041). However, contrast-enhanced brain magnetic-resonance imaging (MRI) showed more brain lesions than [ 18 F]F-FAPI PET/CT (56 vs.34, P = 0.002). Compared with the [ 18 F]F-FDG-based TNM stage, the [ 18 F]F-FAPI-based TNM stage was upgraded in six patients (17.6%). Conclusions [ 18 F]F-FAPI PET/CT showed a very high detection rate for primary LAD. In addition, 18 F-FAPI PET/CT demonstrated clearer tumor delineation and more lesions than [ 18 F]F-FDG PET/CT, especially in lymph nodes, the brain, and pleura. Therefore, [ 18 F]F-FAPI had an advantage over [ 18 F]F-FDG for primary staging of LAD. However, brain MRI could identify more and smaller lesions than [ 18 F]F-FAPI PET/CT.