ESTRO 2021 Abstract Book

S537

ESTRO 2021

both of the nature and content of patient provider communications. In some instances the relationship with clinicians can be improved providing patients with easy access to and an increased amount of communication with clinicians and increased information sharing. 4. Decision-making Confidence in decision-making increased with health literacy, especially amongst those with a low baseline level of confidence. An increase in active discussions around decision-making with both providers and social support networks. 5. Facilitation Strategies need to consider how health literacy could be improved. The use of mHealth apps are advised. Evidence of a digital divide is highlighted in some studies and shows people with low levels of health literacy were less likely to own or use a mobile phone for health related purposes (Kim et al., 2018) and in some cases there was evidence of a need to offer one to one support and explanation to participants in how to use the apps. Content needs to be designed around evidence-based health literacy strategies including: learn about the users; inclusion of actionable content; displaying content clearly; organising and simplifying content; engaging with users; and constant evaluation and revision of the site. Reference: Nutbeam, D. (2017) Discussion paper on promoting , measuring and implementing health literacy : Implications for policy and practice in non-communicable disease prevention and control WHO GCM / NCD Working Group 3 . 3 2017 Don Nutbeam Professor of Public Health , School of, (March), pp. 1–29. Abstract Text Proton therapy as cancer treatment provide a more favourable dose distribution compared to x-rays. While the physical characteristics of proton radiation have been the aim of intense research, less focus has been on the actual biological response proton irradiation gives rise to. Protons have a higher radiobiological effect (RBE), but RBE is a complex quantity, depending on both biological and physical parameters. Currently in proton therapy a constant RBE of 1.1 is generically used, meaning that a given proton dose is expected to be equivalent to a 10% higher x-ray dose for all tumors and tissues. However, whether this is an adequate solution is under debate. There is currently no doubt that an RBE of 1.1 is an oversimplification of the actual biological response to proton irradiation, but most of the data to enlighten this 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. RBE is often established as measured by cell death, but emerging evidence also demonstrate an altered response in the surviving cells. This differential biological effect is not only relevant in the tumour, but also in the normal tissue. Current research in proton radiobiology is, in addition to the RBE, focusing on the molecular tissue response, and on the signalling pathways. Studies indicate a differential DNA damage response (DDR) following high LET irradiation, with the double strand breaks DSB preferentially repaired by homologous recombination (HR), leading to an increased level of unrepaired damage. SP-0671 The clinical impact of a variable RBE H. Paganetti 1 1 Massachusetts General Hospital & Harvard Medical School, Department of Radiation Oncology, Boston, USA Abstract Text When treating patients with proton therapy, radiation oncology currently applies a constant and generic value for the relative biological effectiveness (RBE) of 1.1 to scale doses relative to photon therapy. This value was chosen conservatively mainly to ensure tumor coverage. As it is well known that the RBE depends on dose, endpoint, and linear energy transfer, there is an ongoing debate whether clinical practice should be revised. The use of a constant value has been challenged particularly when considering normal tissue constraints. Unfortunately, the majority of experimental RBE data are from clonogenic cell survival in vitro with unclear relevance for in vivo endpoints. This presentation will first discuss the relationship between RBE values measured in vitro and RBE effects expected in patients for tumors and normal tissue toxicities. While the RBE for clonogenic cell survival is likely related to tumor cell kill, it fails to account for the tumor environment such as vascular damage or immune response. While, for now, a conservative RBE seems appropriate for prescription doses, more realistic RBE estimates should be applied when retrospectively analyzing tumor control data. Even though we expect smaller variations in the underlying cell population for normal tissues as compared to tumors, the mechanisms leading to normal tissue toxicities are even more complex due to the variety of toxicities but also due to the potentially higher impact of vascular damage as compared to cell death in the parenchyma. Furthermore, one has to consider functional sub-units and the often vastly different dose distribution between photons and protons. Next, the clinical evidence for a variable RBE in proton therapy patient outcomes will be reviewed. Specifically, data on brain and brainstem necrosis as well as rib fractures and lung density changes will be discussed. Focus will be on the limitations when analyzing RBE effects in clinical data. Finally, the presentation will outline potential paths forward for biological treatment planning and outcome assessment based on empirical RBE models in proton therapy. The potential of improving treatments for proton therapy patients using linear energy transfer based optimization will be demonstrated. Symposium: RBE in proton therapy SP-0670 RBE from the biological site B.S. Sørensen 1 1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark

SP-0672 RBE from a physical point of view