Despite the small sample size (four patients; age range, 41–71 years; median age, 52 years; all women; stage III or IV breast cancer), this study demonstrates the potential of [18F]F-FTT PET to non-invasively quantify PARP1 expression and provides early evidence of using this modality to assess PARPi drug-target engagement, indicating its potential as a biomarker for treatment with PARPis. Here, PARP1 is linked to breast carcinoma.