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.

Trial registration: EudraCT, EudraCT 2022-001867-29. Registered 02 November 2022.