ESTRO 2025 - Abstract Book

S3478

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

ESTRO 2025

1880

Proffered Paper Validated energy layer-wise beam deflection compensation method for scanned proton beams in the static magnetic field of an in-beam MR scanner Krishna Godino Padre 1,2 , Franciska Lebbink 1,2 , Marisa Cobanaj 1,3 , Erik Traneus 4 , Michael Schürer 1,2 , Hermann Fuchs 5 , Esther Troost 1,3,6 , Jörg Pawelke 1,3 , Aswin Hoffmann 1,3,6 , Felix Horst 1,3 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 National Center for Tumor Diseases (NCT/UCC), Dresden, Germany; German Cancer Research Center (DKFZ), Heidelberg, Germany, Medizinische Fakultät and University, Hospital Carl Gustav Carus Technische Universität Dresden, Germany; Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Dresden, Germany. 3 Institute of Radiooncology-OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany. 4 Department of Research, RaySearch Laboratories AB, Stockholm, Sweden. 5 Division of Medical Physics, Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria. 6 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany Purpose/Objective: The integration of magnetic resonance imaging with proton therapy is expected to increase the targeting accuracy of proton therapy [1]. However, the static MR magnetic field affects the proton beam trajectory, causing distorted dose distributions. A robust compensation strategy is thus essential to ensure accurate proton delivery in the presence of the magnetic field. Proton pencil beam scanning allows for adjustment of beam spot positions, which provides an effective method to correct for beam energy dependent magnetic field induced beam deflections. This study investigates the feasibility of layer-wise energy-dependent beam deflection compensation to restore single field uniform dose (SFUD) distributions in the presence of the magnetic field. Material/Methods: A research version of a commercial Monte Carlo (MC)-based proton treatment planning system (TPS) (RayStation, 2023B-IonPG) was used to import the 3D magnetic vector field map of our 0.32T in-beam MR scanner and calculate lateral beam deflections at the Bragg peak depth in water for 15x15 cm 2 monoenergetic fields ranging from 100– 220 MeV. A spot position correction method, based on beam energy, was applied layer-wise for three spread-out Bragg peaks (SOBPs) in water (100-130 MeV, 120-162 MeV, and 160-220 MeV). Verification measurements were conducted in the MR scanner’s magnetic field using an Octavius 1500XDR detector array with different MULTICube phantom plates and a Markus ionization chamber (IC) in a motorized 3D water phantom. The method’s accuracy was validated through absolute dose comparisons and gamma index analysis of measured versus calculated 2D dose distributions at various depths. Results: Dose distributions can be successfully recovered in the presence of the MR magnetic field (Fig. 1). The gamma pass rates were above 95% at a 3%/3mm 10% dose threshold and 3%3mm 5% dose threshold for all SOBPs. Absolute dose measurements were within 2-3% agreement, and range accuracy between measured and calculated SOBPs were within 1 mm as demonstrated in the 100-130MeV SOBP (Fig. 2).