ESTRO 2021 Abstract Book

S315

ESTRO 2021

D 1.1 increased inter-institutional differences in dosimetric parameters by 1-7 Gy(RBE) in the CTVs (Fig.2). Additionally, mean D varRBE deviated from prescribed D 1.1 by up to 5 Gy(RBE) for each anatomical site. For OARs, inter-institutional dose differences remained comparable when using D varRBE instead of D 1.1 and all variable RBE models consistently predicted an increase of D varRBE by 3-9 Gy(RBE) in OARs located at the distal beam edge and for treatment plans with small angles between single incident beams (brain, BoS, pancreas). There, D varRBE also exceeded OAR near-maximum tolerance doses at all centres. The main drivers of inter-centre D varRBE variance were differences in the considered centre- and treatment site-specific α/β as well as the model- specific handling of α/β. When all centres used the same variable RBE model and α/β value, inter-centre differences in D varRBE distributions essentially reduced to the order of those for D 1.1 . Conclusion Clinical D varRBE calculations are readily available at multiple institutions and already inform clinicians on treatment plan safety. However, to ensure future comparability of results between PT centres and to realize the full potential of PT, it remains a critical task to collectively agree on how to utilize variable RBE models in clinical practise.

OC-0419 LET and RBE investigation of various structures for a cohort of proton neuro-oncological patients A. Vaniqui 1 , F. Vaassen 2 , D. Di Perri 3 , I. Compter 2 , W. van Elmpt 2 , M. Unipan 2 1 Maastro, GROW School for Oncology, Maastricht University Medical Centre+, Department of Radiation Oncology, Maastricht, The Netherlands; 2 Maastro, GROW School for Oncology, Maastricht University Medical Centre+, Department of Radiation Oncology , Maastricht, The Netherlands; 3 Cliniques universitaires Saint- Luc, Maastro, GROW School for Oncology, Maastricht University Medical Centre+, Department of Radiation Oncology, Brussels, Belgium