[(68)Ga]FAPI-PET/CT for radiation therapy planning in biliary tract, pancreatic ductal adeno-, and adenoidcystic carcinomas

Biliary-tract-carcinomas (BTC), pancreatic-ductal-adenocarcinomas (PDAC) and adenoidcystic-carcinomas (AC) have in common that they are traditionally treated with large clinical-target-volumes (CTV). The aim of this study is to examine the impact of pretreatment-[(68)Ga]FAPI-PET/CT on target-volume-definition and posttreatment-[(68)Ga]FAPI-PET/CT-response-assessment for BTC-, PDAC- and AC-patients referred to radiation-therapy. All consecutive BTC-, PDAC-, and AC-patients who received pretreatment-[(68)Ga]FAPI-PET/CT±[(18)F]FDG-PET/CT were included from 01.01.2020 to 01.03.2022. MTV and SUV(max) were separately generated based on [(68)Ga]FAPI- and [(18)F]FDG-PET/CT-images. A [(68)Ga]FAPI- and [(18)F]FDG-based-CTV was defined. Treatment-plans were compared. Treatment-response was reassessed by a second [(68)Ga]FAPI-PET/CT and [(18)F]FDG-PET/CT after treatment-completion. Intermodality comparison of lesion-to-background-ratios [SUV(max_lesion)/SUV(mean_background)] for individual timepoints t(1) and t(2) revealed significant higher values for [(68)Ga]FAPI compared to [(18)F]FDG (t(1), p = 0.008; t(2), p = 0.005). Intermodality comparison of radiation-therapy-plans showed that [(68)Ga]FAPI-based planning resulted in D100% = 97.2% and V95% = 98.8% for the [(18)F]FDG-MTV. [(18)F]FDG-based-planning resulted in D100% = 35.9% and V95% = 78.1% for [(68)Ga]FAPI-MTV. [(18)F]FDG-based-planning resulted only in 2 patients in V95% > 95% for [(68)Ga]FAPI-MTV, and in 1 patient in D100% > 97% for [(68)Ga]FAPI-MTV. GTV-coverage in terms of V95% was 76.4% by [(18)F]FDG-based-planning and 99.5% by [(68)Ga]FAPI-based-planning. Pretreatment [(68)Ga]FAPI-PET/CT enhances radiation-treatment-planning in this particular group of patients. While perilesional and tumoral follow-up [(18)F]FDG-uptake behaved uniformly, perilesional and tumoral reaction may differ in follow-up [(68)Ga]FAPI-imaging. Complementary [(68)Ga]FAPI- and [(18)F]FDG-imaging enhance treatment-response-assessment.