ESTRO 2021 Abstract Book

S314

ESTRO 2021

Conclusion The EPTN study provides deep insights into actual clinical RBE practice. It reveals a two-fold strategy across European PT centres: adhering to a constant RBE prescription of 1.1 while actively considering RBE variability in OARs to reduce the risk of toxicity. The clear demand from PT centres for more clinical data calls for future clinical trials and active research to resolve and mitgate the current RBE discrepancies and uncertainties in translating RBE models into clinical effects. OC-0418 European multi-centric study on variable proton RBE dose calculations for multiple anatomical sites C. Hahn 1,2,3,4 , J. Ödén 5 , A. Dasu 6,7 , A. Vestergaard 8 , J. Folsted Kallehauge 8 , C. Pardi 9 , F. Bourhaleb 9 , A. Leite 10 , L. de Marzi 10,11 , E. Smith 12,13 , A. Aitkenhead 12,13 , M. Merchant 12,13 , K. Kirkby 12,13 , E.G.C. Troost 1,2,3,14,15 , A. Lühr 1,3,4,14 1 OncoRay – National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany; 2 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany; 3 German Cancer Consortium (DKTK), Partner Site Dresden, and German Cancer Research Center (DKFZ), Heidelberg, Germany; 4 Medical Physics and Radiotherapy, Faculty of Physics, TU Dortmund University, Dortmund, Germany; 5 RaySearch Laboratories AB, Research, Stockholm, Sweden; 6 The Skandion Clinic, Radiation Oncology, Uppsala, Sweden; 7 Medical Radiation Sciences, Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden; 8 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark; 9 I-SEE, Internet-Simulation Evaluation Envision, Torino, Italy; 10 Institut Curie, PSL Research University, Radiation Oncology Department, Proton Therapy Centre, Centre Universitaire, 91898 Orsay, France; 11 Institut Curie, PSL Research University, University Paris Saclay, Inserm LITO, 91898 Orsay, France; 12 Christie Medical Physics and Engineering, The Christie NHS Foundation Trust, Manchester, United Kingdom; 13 Division of Cancer Sciences, School of Medical Sciences, Faculty of Biology, Medicine and Health, The University of Manchester, Manchester, United Kingdom; 14 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology – OncoRay, Dresden, Germany; 15 National Center for Tumor Diseases (NCT), Partner Site Dresden, Germany: German Cancer Research Center (DKFZ), Heidelberg, Germany; Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany , and; Helmholtz Association / Helmholtz-Zentrum Dresden - Rossendorf (HZDR), Dresden, Germany Purpose or Objective An increasing number of publications has reported on a variable relative biological effectiveness (RBE) in proton therapy (PT) challenging the clinically applied constant RBE. Since no standard for clinical variable RBE calculations exists, many PT centres independently account for RBE variability. This multi-centric study compared variable RBE dose calculations from different PT institutions towards developing a consistent clinical RBE implementation. Materials and Methods Each of six European PT institutions generated clinically acceptable robust pencil beam scanning treatment plans for five cases, comprised of the sites: brain, base of skull (BoS), head and neck, pancreas and prostate. For plan optimisation, centres used their beam model, treatment planning software and an RBE of 1.1 ( D 1.1 ). All centres used the same patient-specific prescriptions, fractionation, clinical goals and beam directions. Then, each centre recalculated the corresponding variable RBE weighted dose ( D varRBE ) according to their local procedure, including choice of variable RBE model and parameters. Here, an analysis of differences in volume- histogram parameters in clinical target volumes (CTVs) and organs at risk (OARs) is presented for all five anatomical sites and three of the six centres. Results Each of the three centres applied a different in-vitro data-based linear energy transfer (LET) driven variable RBE model, including tissue-specific radiosensitivity (α/β) as model parameter (McNamara and Wedenberg models) or without α/β dependency (McMahon model), respectively (Fig.1). For the CTVs, volume-histogram parameters of D 1.1 and LET were comparable among the centres for each patient. Considering D varRBE instead of