Head-to-head Intra-individual Comparison of 68Ga-FAPI and 18F-FDG PET/CT in Patients with Bladder Cancer

Abstract Aim/Purpose Fibroblast activation protein-(FAP)-ligands, a novel class of tracers for PET/CT imaging, demonstrated promising results in previous studies in various malignancies compared to standard 18 FFDG PET/CT. 68 Ga-labeled fibroblast activation protein inhibitor-( 68 GaGa-DOTA-FAPI)-PET/CT impresses with sharp contrasts in terms of high tumor uptake and low background noise leading to clear delineation. 18 FFDG PET/CT has limited accuracy in bladder cancer due to high background signal. Therefore, we sought to evaluate the diagnostic potential of 68 GaFAPI in patients with bladder cancer. Material and Methods This retrospective analysis consisted of 8 patients (median age 66), 7 of whom underwent both 68 GaFAPI and 18 FFDG PET/CT scans with a median time interval of 5 days (range 1–20 days). Quantification of tracer uptake was determined with SUV max and SUV mean . Furthermore, the tumor-to-background ratio (TBR) was derived by dividing the SUV max of tumor lesions by the SUV max of adipose tissue, skeletal muscle, and blood pool. Results Overall, 31 metastases were detected in five patients including lymph node metastases (n = 23), bone metastases (n = 4), lung metastases (n = 3), and a peritoneal metastasis (n = 1). In one patient, 68 GaFAPI demonstrated significant uptake in the primary tumor located in the bladder wall. 68 GaFAPI-PET/CT demonstrated significantly higher uptake compared to 18 FFDG PET/CT with higher mean SUV max (8.2 vs. 4.6; p = 0.01). Furthermore, 68 GaFAPI detected additional 30% (n = 9) lesions, missed by 18 FFDG. TBR demonstrated favorable uptake for 68 GaFAPI in comparison to 18 FFDG. Significant differences were determined with regard to metastasis/blood pool ( 68 GaFAPI 5.3 vs 18 FFDG 1.9; p = 0.001). Conclusion 68 GaFAPI-PET/CT is a promising diagnostic radioligand for patients with bladder cancer. This first described analysis of FAP-ligand in bladder cancer revealed superiority over 18 FFDG in a small patient cohort. Thus, this so far assumed potential has to be confirmed and extended by larger and prospective studies.