ESTRO 2021 Abstract Book

S313

ESTRO 2021

Proffered papers: Proffered papers 26: Treatment planning for proton therapy

OC-0417 Is a variable proton RBE considered in clinical practice? – ESTRO survey among 25 proton centres L. Heuchel 1 , C. Hahn 1,2,3 , J. Pawelke 2,4 , B. Singers Sørensen 5 , M. Dosanjh 6,7 , A. Lühr 1,2 1 TU Dortmund, Faculty of Physics, Medical Physics and Radiotherapy, Dortmund, Germany; 2 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; 3 Technische Universität Dresden, Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Dresden, Germany; 4 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology – OncoRay, Dresden, Germany; 5 Department of Experimental Clinical Oncology and Danish Center for Particle Therapy, DCPT, Aarhus University Hospital, Aarhus, Denmark; 6 University of Oxford, Department of Physics, Oxford, United Kingdom; 7 CERN, -, Geneva , Switzerland Purpose or Objective Currently in the clinic, proton beam therapy uses a constant relative biological effectiveness (RBE) of 1.1 compared to photons for both tumours and normal tissue. However, in addition to pre-clnical knowledge, recent clinical evidence indicates the variability of the proton RBE as a function of, e.g., linear energy transfer (LET), which may cause unforeseen over- and under-dosage in patients. Within the framework of ESTRO’s European Particle Therapy Network (EPTN), a survey was designed to evaluate how proton therapy (PT) centres consider RBE in current clinical practice. This work analyses the responses and presents the current status on clinical RBE in Europe. Materials and Methods In 2020, clinical and medical physics directors of 25 European PT centres treating patients were asked to provide a contact person to answer an online questionnaire. In 15 centres the questions were answered by physicists, in eight by clinicians and in one by both. The questionnaire consisted of 38 questions and addressed six topics affecting clinical RBE: treatment field arrangement, robust optimisation, variability of RBE in treatment planning, estimation of LET and RBE for patient treatment, RBE consideration for patient follow-up and future improvements. If desired, single questions could be skipped. Results Between June and November 2020, 24 centres (96%) responded, demonstrating a high interest in the topic. All centres are following the current clinical guidelines and prescribed a constant RBE of 1.1 (Fig. 1a). However, all centres, except one which is performing eye treatment, applied various measures to counteract a potentially variable RBE (Fig. 1b). Nearly all centres (21/22, 95%) considered RBE variability in organs at risk (OAR) (Fig. 2a). The most common measure was to select treatment field arrangements that mitigate increased risk of toxicity due to potentially elevated RBE (Fig. 2b). Particularly beams stopping inside or in front of an OAR were prevented or minimized. Most of centres performed some patient-specific calculations of, e.g., LET or variable RBE. While most of these calculations were performed retrospectively to study possible clinical RBE effects, some centres used patient-specific variable RBE distributions for treatment planning. For the future, many respondents asked, with respect to RBE variability, for more (clinical) evidence. Furthermore, standardized calculations of LET and RBE available in commercial treatment planning systems are being requested.