ESTRO 36 Abstract Book

S86 ESTRO 36 _______________________________________________________________________________________________

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

Conclusion Fast CBCT imaging can be safely used for ES-NSCLC tumors with tumor movement amplitude < 1cm. In 73.7 % of the cases there is no image quality loss and even more, in 18.8 % of the cases IQ of the fast scan is preferred compared to the standard scan. (1) Rit, S., et al., Comparative study of respiratory motion correction techniques in cone-beam computed tomography. Radiotherapy and Oncology, 2011. 100(3): p. 356-359 SP-0167 The ESTRO initiative on biological effects of particle therapy B.S. Sørensen 1 1 Aarhus University Hospital, Exp. Clin. Oncology, Aarhus C, Denmark Particle therapy as cancer treatment, with either protons or heavier ions, provide a more favourable dose distribution compared to x-rays. While the physical characteristics of particle radiation have been the aim of intense research, less focus has been on the actual biological responses particle irradiation gives rise to. One of the biggest challenges for the radiobiology is the RBE, with an increasing concern that the clinical used RBE of 1.1 is an oversimplification, as RBE is a complex quantity, depending on both biological and physical parameters, as dose, LET, biological models and endpoints. Most of the available RBE data is in vitro data, and there is very limited in vivo data available, although this is a more appropriate reflection of the complex biological response. There is a need for a systematic, large-scale setup to thoroughly establish the RBE in a number of different models, in a clinical relevant fractionated scheme. The aim of the ESTRO initiative is to form a network of the research and therapy facilities. This would open for the possibility of standardising radiobiological experiments, and coordinating the research in order to deliver the needed experimental data. SP-0168 RBE of protons B. Jones 1 1 Jones Bleddyn, CRUK-MRC Oxford Institute- Department of Oncology, Oxford, United Kingdom Introduction . Increasing clinical use of proton therapy (PT) is not simply an extension of photon radiotherapy (RT), but requires more detailed knowledge of clinical physics and radiobiology in order to achieve optimal outcomes. A critical difference is that megavoltage RT has linear energy transfer (LET) of around 0.22 keV.µm -1 , but LET further increases towards and within proton Bragg peaks. ‘Spread-out’ Bragg peaks (SOBP), depending on their volume, normally have LET of 1-2 keV.µm -1 , but Symposium: Novel approaches in particle biology