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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-[68Ga]FAPI-PET/CT on target-volume-definition and posttreatment-[68Ga]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-[68Ga]FAPI-PET/CT±[18F]FDG-PET/CT were included from 01.01.2020 to 01.03.2022. MTV and SUVmax were separately generated based on [68Ga]FAPI- and [18F]FDG-PET/CT-images. A [68Ga]FAPI- and [18F]FDG-based-CTV was defined. Treatment-plans were compared. Treatment-response was reassessed by a second [68Ga]FAPI-PET/CT and [18F]FDG-PET/CT after treatment-completion. Intermodality comparison of lesion-to-background-ratios [SUVmax_lesion/SUVmean_background] for individual timepoints t1 and t2 revealed significant higher values for [68Ga]FAPI compared to [18F]FDG (t1, p = 0.008; t2, p = 0.005). Intermodality comparison of radiation-therapy-plans showed that [68Ga]FAPI-based planning resulted in D100% = 97.2% and V95% = 98.8% for the [18F]FDG-MTV. [18F]FDG-based-planning resulted in D100% = 35.9% and V95% = 78.1% for [68Ga]FAPI-MTV. [18F]FDG-based-planning resulted only in 2 patients in V95% > 95% for [68Ga]FAPI-MTV, and in 1 patient in D100% > 97% for [68Ga]FAPI-MTV. GTV-coverage in terms of V95% was 76.4% by [18F]FDG-based-planning and 99.5% by [68Ga]FAPI-based-planning. Pretreatment [68Ga]FAPI-PET/CT enhances radiation-treatment-planning in this particular group of patients. While perilesional and tumoral follow-up [18F]FDG-uptake behaved uniformly, perilesional and tumoral reaction may differ in follow-up [68Ga]FAPI-imaging. Complementary [68Ga]FAPI- and [18F]FDG-imaging enhance treatment-response-assessment. [ABSTRACT FROM AUTHOR]