Fibroblast Activation Protein (FAP) specific PET for advanced target volume delineation in Glioblastoma

Ziel/Aim The purpose of this analysis is to assess target volume alterations in glioblastoma (GBM) when contouring is based on PET-CT with Ga-68-radiolabeled ligands of Fibroblast Activating Protein, FAPI-02 and FAPI-04, versus conventional pretreatment imaging with MRI.

Methodik/Methods Fourteen patients with GBM received an additional FAP-specific PET prior to treatment. Three different PET-GTVs were created using a 5-, 7- and 10-fold threshold of increased uptake compared to normal tissue (FAPx5, FAPx7, FAPx10). PET-GTVs were corrected for false positive enhancement and then compared to MRI-GTVs created according to EORTC guidelines.

Ergebnisse/Results No complications were observed. Creating GTVs based on MRI resulted in a median volume of 33.8 cc while GTVs based on FAPx5, x7 and x10 resulted in volumes of 49.6 cc, 33.7 cc and 23.8 cc respectively. Sizes of FAP-specific PET-GTVs were significantly different from MRI-GTVs for FAPx5 (p = 0.022) but not FAPx7 and FAPx10. As MRI- and FAP-specific PET-GTVs were not congruent, adding FAP-specific PET-GTVs to MRI-GTVs resulted in median volumes of 60.8 cc, 49.3 cc and 43.4 cc for FAPx5, x7 and x10 respectively which corresponds to increases of the MRI-GTV by 79.9 %, 45.9 % and 28.4 %. Increases of the GTV were highly significant for all FAP-specific PET thresholds (p  <  0.001).

Schlussfolgerungen/Conclusions Ga-68-radiolabeled FAP-specific PET for target volume delineation caused no complications and resulted in GTVs containing uptake not covered by MRI-GTVs. Heterogeneous uptake in the tumor might prove useful for biopsy planning given FAP’s importance for invasive growth.