ESTRO 2024 - Abstract Book

S3236

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Marisa Cobanaj 1,2 , Benjamin Gebauer 1,2 , Franciska Lebbink 1,3,4 , Brad Oborn 5 , Jörg Pawelke 1,2 , Armin Lühr 6 , Aswin Hoffmann 1,2,4 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 Institute of Radiooncology-OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany. 3 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. 4 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden Rossendorf, Dresden, Germany. 5 Illawarra Cancer Care Centre (ICCC), Wollongong, NSW, 2500, Australia. 6 Department of Physics, TU Dortmund University, Dortmund, Germany Previous experiments investigating the influence of a 0.32 T perpendicular magnetic field (MF) generated by an MR scanner on the delivery of proton pencil beams revealed deflected beam paths and altered dose spot properties (i.e., size, shape, and orientation) 1 . The strongest effects occurred at the lowest energy of 100 MeV, with a 5.1% reduction in spot size ( S spot ) at the extended isocenter ( IS ext ) of the pencil beam scanning (PBS) system, falling within the acceptable tolerance of ±10% recommended by QA guidelines 2 . The IS ext was determined by the MR-imaging isocenter, 582 mm from the PBS nozzle isocenter ( IS nozzle ) along the central beam axis. Range shifters (RSs) are currently employed to overcome limitations in minimum deliverable proton energy for treatment at shallow depths, by lowering incident energies, and to minimize proton beam scattering in air, by modulating high-energy beams just before the treatment site 3 . This work aims to investigate dose spot variations in presence of the MF for RS applications to (a) reduce the residual range of the incident beam, evaluating whether spot changes remain within the tolerance limit, and to (b) improve the beam quality in presence of an additional air gap between IS nozzle and IS ext , evaluating the impact of thickness and positioning distance of the RS. Purpose/Objective:

Material/Methods:

Three PMMA-based RSs of 40 x 30 cm² with different thicknesses ( t RS ) (A, 6 cm; B, 6.5 cm; C, 5.5 cm) were used in single (i.e., A) or combined (i.e., AB and ABC) configurations. To preliminary compute the mean water equivalent thickness ( WET ), a Giraffe detector was used to measure the range at 80% of the distal dose fall-off ( R 80 ) for four proton energies (115, 150, 190, 220 MeV) without and with RS ( WET RS = R 80,without RS - R 80,with RS ). A prototype MR-integrated proton therapy (MRiPT) system with a horizontal PBS beamline and an open 0.32 T in beam MR scanner was employed for measurements in presence of the vertical MF (i.e., B 0 ). 2D relative lateral dose spot profiles (9 spots, field size 15×15 cm²) were measured with EBT3 films placed at IS ext . To investigate the role of RS in improving beam quality, beam transmission measurements were conducted without RS and with RS placed at three different distances ( d RS ) from IS ext (7, 11, 15 cm) ( Figure 1 ).