Assessment of fully quantitative and simplified methods for analysis of [⁶⁸Ga]Ga-FAPI-46 uptake in patients with pancreatobiliary cancer using LAFOV PET/CT

Purpose: The aim of this study was to validate simplified methods for quantifying [⁶⁸Ga]Ga-FAPI-46 uptake against full pharmacokinetic modeling. Methods: Ten patients with pancreatobiliary cancer underwent a 90-min dynamic PET/CT scan using a long axial field of view system. Arterial blood samples were used to establish calibrated plasma-input function from both continuous arterial sampling and image-derived input function (IDIF). Lesional [⁶⁸Ga]Ga-FAPI-46 kinetics were described using conventional non-linear plasma-input tissue-compartment models. Logan plots using 30–90 min and 30–60 min post-injection (p.i), image-based target-to-whole blood ratio (TBR), mean standardized uptake values (SUVmean) normalized to body weight, lean body mass, and body surface area, at 20–30 min, 60–70 min and 80–90 min p.i were assessed. Results: One patient was excluded due to discontinued scan acquisition and missing arterial sampling. Thirteen tumoral lesions and 11 non-tumoral lesions were included. A reversible 2-tissue-compartment model showed most preferrable fits for all types of [⁶⁸Ga]Ga-FAPI-46 positive lesions. The distribution volume (V_T) results obtained using arterial sampling plasma-input function and those using plasma-IDIF (V_{T_plasma_IDIF}) showed an excellent correlation (Spearman rank correlation coefficient (r_s) = 0.949). Logan V_T using both time intervals were highly correlated with V_{T_plasma_IDIF} (r_s ≥ 0.938). The correlation values with V_{T_plasma_IDIF} for image-based TBR and SUVmean parameters were higher at 80–90 min (r_s ≥ 0.839) and at 60–70 min (r_s ≥ 0.835) p.i than at 20–30 min p.i (r_s ≤ 0.774). Conclusion: Image-based TBR and SUVmean at 60–70 min p.i are suitable for quantifying [⁶⁸Ga]Ga-FAPI-46 uptake.