Tumor-to-blood ratio for assessment of fibroblast activation protein receptor density in pancreatic cancer using [68Ga]Ga-FAPI-04.

PURPOSE: [68Ga]Ga-FAPI PET/CT has been widely used in clinical diagnosis and radiopharmaceutical therapy. In this study, tumor-to-blood ratio (TBR) was evaluated as a powerful tool for semiquantitative assessment of [68Ga]Ga-FAPI-04 tumor uptake and as an effective index for tumors with high FAP expression in theranostics. METHODS: Nine patients with pancreatic cancer underwent a 60-min dynamic PET/CT scan by total-body PET/CT (with a long AFOV of 194 cm) after injection of [68Ga]Ga-FAPI-04. After dynamic PET/CT scan, three patients received chemotherapy and underwent the second dynamic scan to evaluate treatment response. Time-activity curves (TACs) were obtained by drawing regions of interest for primary pancreatic lesions and metastatic lesions. The lesion TACs were fitted using four compartment models by the software PMOD PKIN kinetic modeling. The preferred pharmacokinetic model for [68Ga]Ga-FAPI-04 was evaluated based on the Akaike information criterion. The correlations between simplified methods for quantification of [68Ga]Ga-FAPI-04 (SUVs; tumor-to-blood ratios [TBRs]) and the total distribution volume (Vt) estimates obtained from pharmacokinetic analysis were calculated. RESULTS: In total, 9 primary lesions and 25 metastatic lesions were evaluated. The reversible two-tissue compartment model (2TCM) was the most appropriate model among the four compartment models. The total distribution volume Vt values derived from 2TCM varied significantly in pathological lesions and background regions. A strong positive correlation was observed between TBRmean and Vt from the 2TCM model in pathological lesions (R2=0.92, P<0.001). The relative difference range for TBRmean was 2.1% compared to the reduction rate of Vt in the patients who were treated with chemotherapy. CONCLUSIONS: A strong positive correlation was observed between TBRmean and Vt for [68Ga]Ga-FAPI-04. TBRmean reflects FAP receptor density better than SUVmean and SUVmax, and would be the preferred measurement tool for semiquantitative assessment of [68Ga]Ga-FAPI-04 tumor uptake and as a means for evaluating treatment response.