title: Systematic out-of-field secondary neutron spectrometry and dosimetry in pencil beam scanning proton therapy.
abstract: OBJECTIVE
  Systematic investigation of the energy and angular dependence of secondary neutron fluence energy distributions and ambient dose equivalents values (H*(10)) inside a pencil beam scanning proton therapy treatment room using a gantry.
  
  
  METHODS
  Neutron fluence energy distributions were measured with an extended-range Bonner sphere spectrometer featuring ³He proportional counters, at four positions at 0°, 45°, 90°, and 135° with respect to beam direction and at a distance of 2 m from the isocenter. The energy distribution of secondary neutrons was investigated for initial proton beam energies of 75 MeV, 140 MeV, and 200 MeV, respectively, using a 2D scanned irradiation field of 11 × 11 cm² delivered to a 30 × 30 × 30 cm³ PMMA phantom. Additional measurements were performed at a proton energy of 118 MeV including a 5 cm range-shifter (PMMA), yielding a Bragg peak position similar to that of 75 MeV protons.
  
  
  RESULTS
  Ambient dose equivalent values from 0.3 μSv/Gy (75 MeV; 90°) to 24 μSv/Gy (200 MeV; 0°) were measured inside the treatment room at a distance of 2 m from the isocenter. H*(10) values were lower (by factors of up to 7.2 (at 45°)) at 75 MeV compared to those at 118 MeV with the 5 cm range-shifter. At 0° and 45°, an evaporation peak was found in the measured neutron fluence energy distributions, at neutron energies around MeV, which contributes about 50% to total H*(10) values, for all investigated proton beam energies.
  
  
  CONCLUSIONS
  This study showed a pronounced increase of secondary neutron H*(10) values inside the proton treatment room with increasing proton energy without beam modifiers. For example, in beam direction this increase was about a factor of 50 when protons of 75 MeV and 200 MeV were compared. The existence of a peak of secondary neutrons in the MeV region was demonstrated in beam direction (0°). This peak is due to evaporation neutrons produced in the existing surrounding materials such as those used for the gantry. Therefore, any simulation of the secondary neutrons within a proton treatment room must take these materials into account. In addition, the results obtained here show that the use of a range-shifter increases the production of secondary neutrons inside the treatment room. Using a range-shifter, the higher neutron doses observed mainly result from the higher incident proton energy (118 MeV instead of 75 MeV when no range-shifter was used), due to higher neutron production cross-sections.
authors.affiliation: Institute of Radiation Protection, Helmholtz Zentrum München (HMGU), Neuherberg, Germany.|Department of Physik, Technical University of Munich, Munich, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Trinkl
authors.affiliation: Institute of Radiation Protection, Helmholtz Zentrum München (HMGU), Neuherberg, Germany.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Mares
authors.affiliation: Department of Medical Physics, Ludwig-Maximilians-Universität München, Garching b. München, Germany.
authors.forename: Franz Siegfried
authors.initials: FS
authors.lastname: Englbrecht
authors.affiliation: Department of Physik, Technical University of Munich, Munich, Germany.|Department of Radiation Oncology, Technical University of Munich, Klinikum rechts der Isar, Munich, Germany.
authors.forename: Jan Jakob
authors.initials: JJ
authors.lastname: Wilkens
authors.affiliation: Institute of Radiation Protection, Helmholtz Zentrum München (HMGU), Neuherberg, Germany.
authors.forename: Marek
authors.initials: M
authors.lastname: Wielunski
authors.affiliation: Department of Medical Physics, Ludwig-Maximilians-Universität München, Garching b. München, Germany.
authors.forename: Katia
authors.initials: K
authors.lastname: Parodi
authors.affiliation: Institute of Radiation Protection, Helmholtz Zentrum München (HMGU), Neuherberg, Germany.
authors.forename: Werner
authors.initials: W
authors.lastname: Rühm
authors.affiliation: Department of Medical Physics, Rinecker Proton Therapy Center, Munich, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Hillbrand
mesh_terms: D006801:Humans; D009502:Neutrons; D019047:Phantoms, Imaging; D061766:Proton Therapy; D011522:Protons; D011874:Radiometry; D013057:Spectrum Analysis
chemical_list: D011522:Protons
keywords: Geant4; extended-range Bonner sphere spectrometer; neutron dosimetry; neutron spectrometry; pencil beam scanning; proton radiotherapy