ESTRO 2021 Abstract Book

S1645

ESTRO 2021

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Conclusion These experiments provide several informations about the chemical mechanisms involved after exposure to UHDR and might bring some insight to understand the FLASH effect. First, we showed that the direct effects are not enhanced at UHDR thus suggesting that they are not involved in the FLASH effect. Then, we showed that UHDR induce less SB than CONV under hypoxic conditions consistently with previous experiments published in bacteria as well as many editorials and modelling papers published in the field (Weiss et al, 1974; Pratx & Kapp, 2019). However, in physioxia, no difference in DNA breaks was measured after UHDR and CONV. Assuming that 4% oxygen mimics healthy tissues (no impact) while 0.5% oxygen mimics tumors (protection), these results are opposite to the preclinical results showing the FLASH effect (protection of normal tissue and eradication of tumor). Thus, plasmid irradiation might be useful to understand DNA damage at UHDR but seems barely relevant to investigate the FLASH effect at the biology level. PO-1931 Identifying resistance mechanisms in breast cancer in patient-derived organoids and 3D cell culture S. Classen 1 , E. Rahlf 1 , C. Petersen 2 , K. Rothkamm 1 , M. Toulany 3,4 , N. Cordes 5,6,7,8 , K. Borgmann 1 1 University Medical Center Hamburg-Eppendorf, Laboratory for Radiobiology and Experimental Radiooncology, Hamburg, Germany; 2 University Medical Center Hamburg-Eppendorf, Department of Radiotherapy and Radiation Oncology, Hamburg, Germany; 3 German Cancer Consortium, Partner Site Tuebingen and German Cancer Research Center, Heidelberg, Germany; 4 University of Tuebingen, Division of Radiobiology & Molecular Environmental Research, Department of Radiation Oncology, Tuebingen, Germany; 5 Technische Universität Dresden, National Center for Radiation Research in Oncology, Molecular and Cellular Radiobiology, Dresden, Germany; 6 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology-OncoRay, Dresden, Germany; 7 German Cancer Consortium, Partner Site Dresden: German Cancer Research Center, Heidelberg, Germany; 8 University Hospital Carl Gustav Carus, Technische Universität Dresden, Department of Radiotherapy and Radiation Oncology, Dresden, Germany