Franz S Englbrecht1, Sebastian Trinkl2, Vladimír Mares3, Werner Rühm3, Marek Wielunski3, Jan J Wilkens4, Martin Hillbrand5, Katia Parodi6. 1. LMU Munich, Faculty of Physics, Department of Medical Physics, Am Coulombwall 1, 85748 Garching bei München, Germany. Electronic address: franz.englbrecht@lmu.de. 2. Helmholtz Zentrum München, Institute of Radiation Medicine, Ingolstädter Landstraße 1, 85764 Neuherberg, Germany; Technical University of Munich, Physics Department, James-Franck-Straße 1, 85748 Garching bei München, Germany. 3. Helmholtz Zentrum München, Institute of Radiation Medicine, Ingolstädter Landstraße 1, 85764 Neuherberg, Germany. 4. Technical University of Munich, Physics Department, James-Franck-Straße 1, 85748 Garching bei München, Germany; Technical University of Munich, Department of Radiation Oncology, School of Medicine and Klinikum rechts der Isar, Ismaninger Str. 22, 81675 München, Germany. 5. Rinecker Proton Therapy Center, Schäftlarnstraße 133, 81371 München, Germany. 6. LMU Munich, Faculty of Physics, Department of Medical Physics, Am Coulombwall 1, 85748 Garching bei München, Germany.
Abstract
PURPOSE: To simulate secondary neutron radiation fields that had been measured at different relative positions during phantom irradiation inside a scanning proton therapy gantry treatment room. Further, to identify origin, energy distribution, and angular emission of the secondary neutrons as a function of proton beam energy. METHODS: The FLUKA Monte Carlo code was used to model the relevant parts of the treatment room in a scanned pencil beam proton therapy gantry including shielding walls, floor, major metallic gantry-components, patient table, and a homogeneous PMMA target. The proton beams were modeled based on experimental beam ranges in water and spot shapes in air. Neutron energy spectra were simulated at 0°, 45°, 90° and 135° relative to the beam axis at 2m distance from isocenter for monoenergetic 11×11cm2 fields from 200MeV, 140MeV, 75MeV initial proton beams, as well as for 118MeV protons with a 5cm thick PMMA range shifter. The total neutron spectra were scored for these four positions and proton energies. FLUKA neutron spectra simulations were crosschecked with Geant4 simulations using initial proton beam properties from FLUKA-generated phase spaces. Additionally, the room-components generating secondary neutrons in the room and their contributions to the total spectrum were identified and quantified. RESULTS: FLUKA and Geant4 simulated neutron spectra showed good general agreement with published measurements in the whole simulated neutron energy range of 10-10 to 103MeV. As in previous studies, high-energy (E≥19.6MeV) neutrons from the phantom are most prevalent along 0°, while thermalized (1meV≤E<0.4eV) and fast (100keV≤E<19.4MeV) neutrons dominate the spectra in the lateral and backscatter direction. The iron of the large bending magnet and its counterweight mounted on the gantry were identified as the most determinant sources of secondary fast-neutrons, which have been lacking in simplified room simulations. CONCLUSIONS: The results helped disentangle the origin of secondary neutrons and their dominant contributions and were strengthened by the fact that a cross comparison was made using two independent Monte Carlo codes. The complexity of such room model can in future be limited using the result. They may further be generalized in that they can be used for an assessment of neutron fields, possibly even at facilities where detailed neutron measurements and simulations cannot be performed. They may also help to design future proton therapy facilities and to reduce unwanted radiation doses from secondary neutrons to patients.
PURPOSE: To simulate secondary neutron radiation fields that had been measured at different relative positions during phantom irradiation inside a scanning proton therapy gantry treatment room. Further, to identify origin, energy distribution, and angular emission of the secondary neutrons as a function of proton beam energy. METHODS: The FLUKA Monte Carlo code was used to model the relevant parts of the treatment room in a scanned pencil beam proton therapy gantry including shielding walls, floor, major metallic gantry-components, patient table, and a homogeneous PMMA target. The proton beams were modeled based on experimental beam ranges in water and spot shapes in air. Neutron energy spectra were simulated at 0°, 45°, 90° and 135° relative to the beam axis at 2m distance from isocenter for monoenergetic 11×11cm2 fields from 200MeV, 140MeV, 75MeV initial proton beams, as well as for 118MeV protons with a 5cm thick PMMA range shifter. The total neutron spectra were scored for these four positions and proton energies. FLUKA neutron spectra simulations were crosschecked with Geant4 simulations using initial proton beam properties from FLUKA-generated phase spaces. Additionally, the room-components generating secondary neutrons in the room and their contributions to the total spectrum were identified and quantified. RESULTS: FLUKA and Geant4 simulated neutron spectra showed good general agreement with published measurements in the whole simulated neutron energy range of 10-10 to 103MeV. As in previous studies, high-energy (E≥19.6MeV) neutrons from the phantom are most prevalent along 0°, while thermalized (1meV≤E<0.4eV) and fast (100keV≤E<19.4MeV) neutrons dominate the spectra in the lateral and backscatter direction. The iron of the large bending magnet and its counterweight mounted on the gantry were identified as the most determinant sources of secondary fast-neutrons, which have been lacking in simplified room simulations. CONCLUSIONS: The results helped disentangle the origin of secondary neutrons and their dominant contributions and were strengthened by the fact that a cross comparison was made using two independent Monte Carlo codes. The complexity of such room model can in future be limited using the result. They may further be generalized in that they can be used for an assessment of neutron fields, possibly even at facilities where detailed neutron measurements and simulations cannot be performed. They may also help to design future proton therapy facilities and to reduce unwanted radiation doses from secondary neutrons to patients.
Authors: Olivier Van Hoey; Liliana Stolarczyk; Jan Lillhök; Linda Eliasson; Natalia Mojzeszek; Malgorzata Liszka; Ali Alkhiat; Vladimir Mares; François Trompier; Sebastian Trinkl; Immaculada Martínez-Rovira; Maite Romero-Expósito; Carles Domingo; Ondrej Ploc; Roger Harrison; Pawel Olko Journal: Front Oncol Date: 2022-09-08 Impact factor: 5.738