ESTRO 2025 - Abstract Book

S506

Clinical - Breast

ESTRO 2025

After contouring, PET/CTs and contours were imported into a BgRT-capable Treatment Planning System (TPS) (RefleXion Medical, Hayward CA). A simulation tool within the TPS was used to convert the diagnostic PET images into simulated BgRT planning PET images to account for differences in system sensitivity, acquisition time, reconstruction method and geometry between the diagnostic PET/CT system and the PET-LINAC. The mean AC (ACmean) was calculated for each lesion’s GTV, PTV, BTZ and BTZ shell, and the maximum AC (ACmax) was measured for GTV and BTZ shell on the diagnostic PET images in RTx. AC and NTS values were also measured on the simulated RefleXion planning PET images in the RefleXion TPS. The BTZ’s distance to nearest organ at risk (OAR) were recorded. Results: Five FES-avid lesions met the BgRT constraints for NTS and AC for plan creation in four patient datasets. For the BgRT eligible lesions, average GTV was 9.0 ± 9.3cc, with a PTV of 26.1 ± 21.6cc. The diagnostic PET PTV ACmean, ER+ lesion ACmean, derived AC and NTS on converted PET images average and standard deviation were 4.7 ± 1.1kBq/ml, 7.5 ± 1.9kBq/ml, 6.63 ± 2.23 kBq/ml and 8.98 ± 2.28, respectively.

Conclusion: This initial study demonstrates sufficient signal intensity metrics for BgRT planning of ER+ breast lesions with FES and constitutes preliminary evidence supporting the feasibility of FES-guided BgRT.

Keywords: radiotracer, biology guided-radiotherapy

References: Shirvani, Shervin M et al. “Biology-guided radiotherapy: redefining the role of radiotherapy in metastatic cancer.” The British Journal of Radiology vol. 94,1117 (2021)

2053

Digital Poster Stereotactic Body Radiation Therapy as a primary option for inoperable breast cancer: initial safety and efficacy results Ines Ollinger 1 , Isabela Gaztelu 1 , Gema Muñiz 2 , Roberto De Haro 1 1 Radiation Oncology, Hospital Universitario Virgen del Rocio, Seville, Spain. 2 Medical Physics, Hospital Universitario Virgen del Rocio, Seville, Spain Purpose/Objective: Surgical intervention is generally central to breast cancer treatment; however, providing curative treatment becomes challenging when surgery is not feasible due to patient conditions or preferences. This study explores Stereotactic Body Radiation Therapy (SBRT) as an approach to improve local control (LC) and limit distant metastasis (DM) in breast cancer patients. We aim to evaluate SBRT’s safety and effectiveness as a primary treatment option for patients for whom surgery is not viable. Material/Methods: Fourteen female breast cancer patients deemed ineligible for surgery were prospectively evaluated. SBRT was administered at a dose of 40 Gy across 5 fractions using a breast stereotactic prototype equipped with rib