[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-[Ga]FAPI-PET/CT on target-volume-definition and posttreatment-[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-[Ga]FAPI-PET/CT±[F]FDG-PET/CT were included from 01.01.2020 to 01.03.2022. MTV and SUV were separately generated based on [Ga]FAPI- and [F]FDG-PET/CT-images. A [Ga]FAPI- and [F]FDG-based-CTV was defined. Treatment-plans were compared. Treatment-response was reassessed by a second [Ga]FAPI-PET/CT and [F]FDG-PET/CT after treatment-completion. Intermodality comparison of lesion-to-background-ratios [SUV/SUV] for individual timepoints t and t revealed significant higher values for [Ga]FAPI compared to [F]FDG (t, p = 0.008; t, p = 0.005). Intermodality comparison of radiation-therapy-plans showed that [Ga]FAPI-based planning resulted in D100% = 97.2% and V95% = 98.8% for the [F]FDG-MTV. [F]FDG-based-planning resulted in D100% = 35.9% and V95% = 78.1% for [Ga]FAPI-MTV. [F]FDG-based-planning resulted only in 2 patients in V95% > 95% for [Ga]FAPI-MTV, and in 1 patient in D100% > 97% for [Ga]FAPI-MTV. GTV-coverage in terms of V95% was 76.4% by [F]FDG-based-planning and 99.5% by [Ga]FAPI-based-planning. Pretreatment [Ga]FAPI-PET/CT enhances radiation-treatment-planning in this particular group of patients. While perilesional and tumoral follow-up [F]FDG-uptake behaved uniformly, perilesional and tumoral reaction may differ in follow-up [Ga]FAPI-imaging. Complementary [Ga]FAPI- and [F]FDG-imaging enhance treatment-response-assessment.