title: Secondary neutron dose during proton therapy using spot scanning.
abstract: OBJECTIVE
  During proton radiotherapy, secondary neutrons are produced by nuclear interactions in the material in the beam line before and after entering the patient. The dose equivalent deposited by these neutrons is usually not considered in routine treatment planning. In this study, we estimated the neutron dose in patients from a spot scanning beam line by performing measurements and Monte Carlo simulations.
  
  
  METHODS
  Measurements of the secondary neutron dose were performed during irradiation of a water phantom with 177-MeV protons using a Bonner sphere and CR39 etch detectors. Additionally, Monte Carlo simulations were performed using the FLUKA code.
  
  
  RESULTS
  A comparison of our measurements with measurements taken at a beam line using the scatter foil technique shows a dose advantage of at least 10 for the spot scanning technique. In the region of the Bragg peak, the neutron dose equivalent can reach for a medium-sized target volume approximately 1% of the treatment dose. Neutron doses expected in healthy tissues of the patient (in the not-treated volume) are for large and medium target volumes, approximately 0.004 Sv and 0.002 Sv per treatment Gy, respectively.
  
  
  CONCLUSIONS
  We conclude from the measurements and simulations that the dose deposited by secondary neutrons during proton radiotherapy using the spot scanning technique can be neglected in the treatment region. In the healthy tissue, the dose coming from neutrons (0.002 Sv per treatment Gy) is approximately a factor of two larger than during photon treatment (0.001 Sv). These contributions to the integral dose from neutrons are still very low when compared to the dose sparing that can be achieved by using a proton beam instead of photons.
authors: Division of Medical Physics, Department of Radiation Oncology and Nuclear Medicine, City Hospital Triemli, CH-8063 Zürich, Switzerland. uwe.schneider@psi.ch
authors: Uwe
authors: U
authors: Schneider
authors: Stefano
authors: S
authors: Agosteo
authors: Eros
authors: E
authors: Pedroni
authors: Jürgen
authors: J
authors: Besserer
mesh_terms: D002138:Calibration; D006801:Humans; D009010:Monte Carlo Method; D009502:Neutrons; D017785:Photons / Q000627:therapeutic use*; D055585:Physical Phenomena; D010825:Physics; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage