ESTRO 2025 - Abstract Book

S4004

Radiobiology - Tumour radiobiology

ESTRO 2025

2666

Proffered Paper FLASH radiotherapy induces a differential immune response between breast tumours and healthy lungs Adrien Arrigo 1 , Louize Brants 1 , Paul Meijnders 1,2 , Dirk Verellen 1,2 , Philip Poortmans 1,2 , An Wouters 3 , Alessia Gasparini 1,2 , Steven van Laere 4 , Pierre Montay-Gruel 1,2 1 Antwerp Research in Radiation Oncology (AReRO), Centre for Oncological Research (CORE), MIPRO, University of Antwerp, Antwerp, Belgium. 2 Radiation Oncology department, Iridium Netwerk, Wilrijk, Belgium. 3 Centre for Oncological Research (CORE), MIPRO, University of Antwerp, Antwerp, Belgium. 4 MIPRO, University of Antwerp, Antwerp, Belgium Purpose/Objective: Breast cancer stands as the primary cause of cancer-related mortality among women, resulting in approximately 685,000 fatalities globally each year. 75% of breast cancer patients undergo radiotherapy (RT) as part of their treatment regimen. While RT is a major anti-cancer treatment modality, it can be limited by its side effects to surrounding healthy organs. By reducing side effects in the lungs and heart, FLASH-RT could benefit breast cancer patients, and allow dose escalation for its most aggressive subtypes. Our study aims at studying the response of triple negative breast cancer (TNBC) lungs and heart to different regimens of FLASH-RT. Material/Methods: Female balb/c mice were injected subcutaneously or in the mammary fat pad with 4T1 cells and irradiated with a single fraction of 14 or 20 Gy or with fractionated regimens of 5 x 5.2 or 10 x 3 Gy, CONV (0.234 Gy/s) or FLASH-RT (7.69x10 6 Gy/s). For lung and heart response, C57BL/6 mice received 17.5 Gy CONV or FLASH-RT to the thorax. Tumor and lung samples were harvested 24 hours, 5 days, 2- and 14-weeks post-irradiation or at humane endpoint for flow cytometry, histology and RNA-sequencing analyses. Echocardiographies were performed at baseline, and every other month to follow radiation-induced heart toxicities. Results: Every dose and fractionation regimen showed isoefficacy of FLASH and CONV-RT. 24h post-FLASH-RT, RNA sequencing data revealed a significant transcription upregulation of genes encoding chemokines, chemokine receptors and immune response modulators. Higher CD45+ cell infiltration was observed after FLASH-RT, but with no difference in CD4+ and CD8+ T-cells. Tumor-associated macrophage populations is under characterization. Unlike tumours, a downregulation of chemokine and immune response modulator genes was found in lungs 24 hours post FLASH-RT, followed after 2 weeks, by a downregulation of chemokine receptor encoding genes. Moreover, FLASH-RT showed downregulation of macrophage-related genes, associated to their polarization and chemotaxis. Flow cytometry revealed an increase 14 weeks after CONV but not FLASH-RT of M1-M2 polarized macrophages. Heart toxicities are under investigation, no impact of radiation was found at 3 months post-RT.