Fibroblast activation protein inhibitor (FAPi) positive tumour fraction on PET/CT correlates with Ki-67 in liver metastases of neuroendocrine tumours.

Aim: Gallium-68-labelled inhibitors of the fibroblast activation protein (FAPi) enable positron emission tomography/computed tomography (PET/CT) imaging of fibroblast activation. We evaluated if [Ga]Ga-DATA5m.SA.FAPi PET/CT is related to Ki-67 as a marker of tumour aggressiveness in patients with liver metastases of NET.

Methods: Thirteen patients with liver metastases of a histologically confirmed NET who underwent PET/CT with [Ga]Ga-DATA5m.SA.FAPi, [18F]FDG and [Ga]Ga-DOTA-TOC were retrospectively analyzed. PET-positive liver tumour volumes were segmented for calculation of volume, SUVmax and PET-positive tumour fraction (TF). PET parameters were correlated with Ki-67.

Results: FDG correlated positively (rho = 0.543, p < 0.05) and DOTATOC correlated negatively (rho = -0.618, p < 0.05) with Ki-67, the correlation coefficients were in the moderate range. There was no significant correlation between FAPi and Ki-67 (rho = 0.382, p > 0.05). FAPi correlated positively (rho = 0.770, p < 0.01) and DOTATOC correlated negatively (rho = -0.828, p < 0.01) with Ki-67, both significantly with high correlation coefficients. FDG also correlated significantly with Ki-67, with a moderate correlation coefficient (rho = 0.524, p < 0.05). The ratio FAPi:DOTATOC showed a significant and strong correlation with Ki-67 (rho = 0.808, p < 0.01).

Conclusion: The ratio FAPi:DOTATOC might serve as a clinical parameter for the assessment of dedifferentiation and aggressiveness of liver metastases in patients with NET. [Ga]Ga-DATA5m.SA.FAPi might hold potential for identification of high-risk patients. Further studies are warranted to evaluate its prognostic significance in comparison to [F]FDG in patients with NET.