ESTRO 36 Abstract Book

S85 ESTRO 36 2017 _______________________________________________________________________________________________

higher values between 2-10 keV.µm -1 can be found in treatment plans. Methods and Results : Studies concluding that the mid-SOBP relative biological effect (RBE) of protons is 1.1 for all tissues and tumours at all doses per fraction have recently been criticised due to their use of: 1. kilovoltage x-ray controls (which mostly provide RBE values less than 1 and should be excluded), 2. a very limited number of cell lines, 3. a predominance of high doses per fraction (as used for eye melanomas), 4. linear-only fitting (rather than linear quadratic), 5. animal based studies that used only acute reacting tissues (with high α/β ratios), known to show little RBE change with dose per fraction when using fast neutrons (which ionise mostly by forming recoil protons). No classical late reacting (low α/β) tissue RBEs have been published so far: it is these tissues that will influence PT late effects for important normal tissues within the PTV and closely around it. Of prime concern is neurological tissue with α/β of 2 Gy. Using a scaling model based on the original work of Wilkens & Oelfke, but with added saturation effects for increases in both α and β with LET, figures 1 and 2 shows the predicted RBEs in the range of LET normally in the SOBP (1-2keV.µm -1 ) and the general increase in RBE with LET and decrease of RBE with dose per fraction; at higher values of LET (2-10) further increases in RBE occur, in some cases to beyond 2 at LETs of 6-10. Outside the brain, other normal tissue types may carry lesser importance so that, for example, a slightly raised RBE in muscle may not produce enhanced late effects in a very confined volume, as may serially organised tissues such as lung and liver, but cardiac tissue, bowel and kidney remain at risk depending on the volume irradiated. One intriguing aspect is the fall of RBE with increased dose per fraction, especially in tisses with low α/β values, which may encourage the use of carefully estimated hypofractionated total doses, using BED equations with imbedded RBE limits: the RBEmax and RBEmin (respectively reflecting the change in α and β with LET): Figures 1 and 2 show how different α/β ratio bio-systems may behave with the lowest α/β system crossing over to have the lowest RBE at higher doses. Values lower than 1.1 can occur in high α/β systems, with risk of underdosage if a 1.1 RBE is used. Conclusions . There should be no complacency about RBE values, even within SOBP`s: 1.1 is not be appropriate. These higher values may explain some reported adverse toxicities following PT, such as necrosis of the optic chiasm and temporal lobe, and failure to cure some very radiosensitive tumour types with high α/β (lymphomas and many childhood cancers). Comprehensive RBE studies are urgently indicated. References: Jones, B in Cancers (Basle) 2015, 7, 460-480; also, Brit J Radiol, Why RBE must be a variable and not a constant. Published Online: May 05, 2016. Figures 1&2

SP-0169 A small animal tumour model for low-energy laser-accelerated particles J. Pawelke 1,2 , K. Brüchner 3,4 , M. Krause 4,5,6 , E. Leßmann 2 , M. Schmidt 5 , E. Beyreuther 2 1 OncoRay - National Center for Radiation Research in Oncology- Faculty of Medicine and University Hospital Carl Gustav Carus-Technische Universität Dresden, Department of Medical Physics- Laser Radiooncology Group, Dresden, Germany 2 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiation Physics, Dresden, Germany 3 Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Experimental Center, Dresden, Germany 4 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology, Dresden, Germany 5 OncoRay - National Center for Radiation Research in Oncology- Faculty of Medicine and University Hospital Carl Gustav Carus-Technische Universität Dresden, Department of Radiation Oncology, Dresden, Germany 6 German Consortium for Translational Cancer Research DKTK and German Cancer Research Center DKFZ, Dresden Site, Dresden, Germany Introduction: The long-term aim of decveloping laser- based acceleration of protons and heavier ions towards clinical radiation therapy application requires not only substantial technological progress, but also the radiobiological characterization of the resulting ultra- short and ultra-intensive particle beam pulses. Recent in vitro data showed similar effects of laser-accelerated versus conventional proton beams on clonogenic cell survival and DNA double-strand breaks. As the proton energies currently achieved for radiobiological experiments by laser-driven acceleration are too low to penetrate standard tumour models on mouse legs, a small animal tumour model allowing for the penetration of low energy protons (~20 MeV) was developed to further verify the effects in vivo. Methods: The mouse ear tumour model was established for human HNSCC FaDu and human glioblastoma LN229 cells. For this, cells were injected subcutaneously in the right ear of NMRI nude mice and the growing tumours were characterized with respect to growth parameters and histology. After optimizing the number of injected cells and used medium (PBS, Matrigel) the radiation response was studied by 200 kV X-ray irradiation. Furthermore, a proof-of-principle full scale experiment with laser- accelerated electrons was performed to validate the FaDu