ESTRO 2025 - Abstract Book

S505

Clinical - Breast

ESTRO 2025

excellent/good in most patients (60%). Grade (G) 2 acute events were observed in three patients (one radiation dermatitis and two fatigue) and only one G2 late event (fatigue) was observed.

Conclusion: Our findings support a good safety profile of 26Gy/5 schedule and the 24-month cardiac assessment reported no significant differences over time in myocardial function

Keywords: ultra-hypofractionation, heart toxicity

1951

Digital Poster Feasibility study of biology-guided radiotherapy for estrogen receptor positive breast tumors using fluoroestradiol Ottar Schmitz 1 , Dorine de Jong 2 , Grant Gibbard 2 , Karine A Al Feghali 2 , Kathleen M Capaccione 3 1 Product Strategy, RefleXion Medical, Hayward, USA. 2 Medical Affairs, RefleXion Medical, Hayward, USA. 3 Department of Radiology, Columbia University, New York, USA Purpose/Objective: Biology-guided radiotherapy (BgRT) utilizing fluorodeoxyglucose (FDG) signal collected via an on-board positron emission tomography (PET) system integrated in a ring gantry linear-accelerator (LINAC) was cleared last year by the FDA for lung and bone lesions (Shirvani et all, British Journal of Radiology 2021). Two PET metrics, Activity Concentration (AC) and Normalized Target Signal (NTS), must be sufficient for patients to be eligible for BgRT. This study aims to investigate the feasibility of BgRT for Estrogen receptor positive (ER+) breast cancer lesions using Fluoroestradiol (FES). Material/Methods: Diagnostic FES PET/CT images from 10 patients with ER+ lesions were retrospectively selected from one academic institution. PET/CT were contoured in RTx (Mirada Medical, Oxford UK) to define target volumes and activity concentrations and gross tumor volumes (GTV) were defined using a threshold of 2 times the mean of overall AC. Planning tumor volumes (PTVs) and Biology-Tracking Zones (BTZs) were defined by a 5 and 10 mm isotropic GTV expansion, respectively.