ESTRO 2025 - Abstract Book

S613

Clinical - Breast

ESTRO 2025

limiting the radiation dose and achieving prolonged local control in some patients, and allows to postpone or withdraw WBRT.

Keywords: SRT, CNS

4116

Digital Poster Safety and efficacy of trastuzumab deruxtecan and concomitant radiation therapy in breast cancer patients: an international retrospective cohort study Luca Visani 1 , Ivica Ratosa 2,3 , Barbro Linderholm 4 , Rupert Bartsch 5 , Domen Ribnikar 6,3 , Niccolò Bertini 1,7 , Ilaria Bonaparte 1,7 , Dimitar Stefanovski 6,3 , Nika Dobnikar 2 , Magdalena Sojar 2 , Angelika Starzer 5 , Isacco Desideri 1,7 , Emanuela Olmetto 1 , Vieri Scotti 1 , Chiara Mattioli 1,7 , Marianna Valzano 1 , Viola Salvestrini 1 , Carlotta Becherini 1 , Lorenzo Livi 1,7 , Icro Meattini 1,7 1 Radiation Oncology & Breast Unit, Department of Oncology, Azienda Ospedaliero-Universitaria Careggi, Florence, Italy. 2 Division of Radiotherapy, Institute of Oncology, Ljubljana, Slovenia. 3 Faculty of Medicine, University of Ljubljana, Ljubljana, Slovenia. 4 Medical Oncology Unit, Department of Oncology, Sahlgrenska Academy and University Hospital, Gothenburg, Sweden. 5 Clinical Research Unit, Division of Oncology, Department of Medicine I, Medical University of Vienna, Vienna, Austria. 6 Division of Medical Oncology, Institute of Oncology, Ljubljana, Slovenia. 7 Department of Experimental and Clinical Biomedical Sciences "M. Serio", University of Florence, Florence, Italy Purpose/Objective: Trastuzumab deruxtecan (T-DXd) has emerged as the standard treatment for patients with metastatic HER2-positive (HER2+) breast cancer (BC) following disease progression on first-line therapy containing taxanes and trastuzumab. Results from DESTINY-Breast04 have extended the approval of T-DXd to include HER2-low patients. Radiation therapy (RT) is an integral component of multimodal treatment in the metastatic setting, used for either palliative or ablative intent. This retrospective study aims to evaluate the safety of the concurrent use of T-DXd and RT in a consecutive, multicentre international cohort of BC patients. Material/Methods: A retrospective analysis was conducted on patients with metastatic HER2+ or HER2-low BC treated at four leading European institutions. Ablative RT was defined based on a biological dose threshold of a minimum of 50Gy EQD2(10) delivered in no more than 12 fractions, as per the European Society for Radiotherapy and Oncology (ESTRO) OligoCare study. The primary objective was to assess the association between RT administration and adverse events (AEs) greater than grade (G) 2. In all cases, RT was administered within one month prior to cycle 1 or during systemic treatment, without any interruption to T-DXd. Results: Data from 118 consecutive patients treated with T-DXd, with or without RT, were retrospectively evaluated. Thirty three patients received RT immediately before or during T-DXd treatment, amounting to 34 concurrent RT treatments, while 85 patients did not receive RT. The median age was 55 years (range 30–88), with a median follow up of 18 months (range 1–33). The median total RT dose was 34Gy (range 8–48), delivered over a median of 4 fractions (range 1–15). The median EQD2 dose was 64Gy (range 12–104), and the median BED was 76Gy (range 14–125). The central nervous system (51.5%; 17/33) was the most frequently treated site, followed by bone (33.3%; 11/33). Fifteen patients (45.5%) received RT with ablative intent, while 18 patients (54.5%) received palliative RT. The relationship between RT administration and the development of >G2 toxicity was not statistically significant (p=0.79).