title: Neutrons in active proton therapy: Parameterization of dose and dose equivalent.
abstract: OBJECTIVE
  One of the essential elements of an epidemiological study to decide if proton therapy may be associated with increased or decreased subsequent malignancies compared to photon therapy is an ability to estimate all doses to non-target tissues, including neutron dose. This work therefore aims to predict for patients using proton pencil beam scanning the spatially localized neutron doses and dose equivalents.
  
  
  METHODS
  The proton pencil beam of Gantry 1 at the Paul Scherrer Institute (PSI) was Monte Carlo simulated using GEANT. Based on the simulated neutron dose and neutron spectra an analytical mechanistic dose model was developed. The pencil beam algorithm used for treatment planning at PSI has been extended using the developed model in order to calculate the neutron component of the delivered dose distribution for each treated patient. The neutron dose was estimated for two patient example cases.
  
  
  RESULTS
  The analytical neutron dose model represents the three-dimensional Monte Carlo simulated dose distribution up to 85cm from the proton pencil beam with a satisfying precision. The root mean square error between Monte Carlo simulation and model is largest for 138MeV protons and is 19% and 20% for dose and dose equivalent, respectively. The model was successfully integrated into the PSI treatment planning system. In average the neutron dose is increased by 10% or 65% when using 160MeV or 177MeV instead of 138MeV. For the neutron dose equivalent the increase is 8% and 57%.
  
  
  CONCLUSIONS
  The presented neutron dose calculations allow for estimates of dose that can be used in subsequent epidemiological studies or, should the need arise, to estimate the neutron dose at any point where a subsequent secondary tumour may occur. It was found that the neutron dose to the patient is heavily increased with proton energy.
authors.affiliation: Department of Physics, Science Faculty, University of Zurich, Zurich, Switzerland; Radiotherapy Hirslanden AG, Rain 34, Aarau, Switzerland. Electronic address: uwe.schneider@uzh.ch.
authors.forename: Uwe
authors.initials: U
authors.lastname: Schneider
authors.affiliation: Department of Physics, Science Faculty, University of Zurich, Zurich, Switzerland; Radiotherapy Hirslanden AG, Rain 34, Aarau, Switzerland.
authors.forename: Roger A
authors.initials: RA
authors.lastname: Hälg
authors.affiliation: Center for Proton Therapy, Paul Scherrer Institut, Villigen-PSI, Switzerland.
authors.forename: Tony
authors.initials: T
authors.lastname: Lomax
mesh_terms: D000465:Algorithms; D006801:Humans; D008962:Models, Theoretical; D009010:Monte Carlo Method; D009381:Neoplasms, Radiation-Induced; D009502:Neutrons / Q000627:therapeutic use*; D017785:Photons / Q000627:therapeutic use; D061766:Proton Therapy / Q000009:adverse effects / Q000379:methods* / Q000592:standards; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted
keywords: Neutronendosis; Parametrisierung; Proton therapy; Protonentherapie; analytical model; analytisches Modell; neutron dose; parameterization