ESTRO 2024 - Abstract Book

S4639

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

1579

Mini-Oral

NTCP reduction due to novel optimization strategies accounting for RBE variability in proton therapy

Martina Palkowitsch 1,2 , Lisa-Marie Kaufmann 1 , Fabian Hennings 1,2 , Stefan Menkel 3 , Christian Hahn 1,4,5 , Jona Bensberg 4 , Armin Lühr 4 , Annekatrin Seidlitz 1,6,3 , Esther G.C. Troost 1,3,2 , Mechthild Krause 1,3,7 , Steffen Löck 1,3,7 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 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology – OncoRay, Dresden, Germany. 3 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany. 4 TU Dortmund University, Department of Physics, Dortmund, Germany. 5 RaySearch Laboratories AB, Service department, Stockholm, Sweden. 6 National Center for Tumor Diseases NCT, Partner Site Dresden - Germany: German Cancer Research Center DKFZ, Heidelberg, Germany. 7 German Cancer Consortium DKTK, partner site Dresden, and German Cancer Research Center DKFZ, Heidelberg, Germany

Purpose/Objective:

The relative biological effectiveness (RBE) of protons varies along the treatment field. Substantial evidence points to an elevated RBE at the field edge (typically located within healthy tissue), where numerous stopping protons lead to an increased linear energy transfer (LET). However, current clinical practice assumes a constant RBE of 1.1, potentially resulting in undesired side effects. This study explored the potential of novel treatment-planning optimization strategies to mitigate side effects that may result from disregarding RBE variability.

Material/Methods:

We conducted a retrospective analysis on a total of 106 primary brain tumour patients, calculating dose-averaged LET (LET d ), variable RBE-weighted dose (D varRBE ) [1], and normal tissue complication probabilities (NTCP) for 16 relevant treatment-related side effects [2]. The change in NTCP considering a variable RBE compared to a constant RBE (ΔNTCP) was calculated. Furthermore, we performed a correlation analysis to investigate the impact of patient-specific anatomical, clinical, and treatment plan characteristics on ΔNTCP. For three patients, plans with two and three irradiation fields were compared to investigate the influence of the number of fields on ΔNTCP. Based on ΔNTCP values, we identified seven high-risk patients for whom considering RBE variability led to a significant increase in NTCP. For these patients, we implemented two novel optimization strategies [3] aimed at reducing D varRBE to adjacent organs at risk (OARs), consequently lowering NTCP values: We applied penalties either to (i) the track-end fraction (TEopt) or to (ii) elevated LET d in voxels above a dose threshold (LETopt). LET- and TE-objectives were added for OARs whose RBE-induced dose burden led to a significant NTCP increase. Furthermore, we evaluated the impact of the number of irradiation fields on the potential of the optimization strategies to reduce NTCP by comparing a 2- to a 3-field plan. All alternative treatment plans were created using the research treatment planning system RayStation v8.99.30.169 (RaySearch Laboratories AB, Stockholm, Sweden) and were reviewed for clinical acceptance.