title: Measurement of neutron dose equivalent and its dependence on beam configuration for a passive scattering proton delivery system.
abstract: OBJECTIVE
  To measure the neutron dose equivalent per therapeutic proton dose (H/D) in a passive scattering proton therapy system and study its dependence on the proton energy, aperture-to-isocenter distance, spread-out Bragg peak (SOBP) width, and field size.
  
  
  METHODS
  We performed four experiments of varying proton energies, aperture-to-isocenter distances, SOBP widths, and field sizes. Etched track detectors were used to measure the neutron dose equivalent at both an in-field (isocenter, beyond the protons' range) and out-of-field (30 cm lateral to the isocenter) location in air.
  
  
  RESULTS
  For a nonmodulated beam with all the protons stopping in the aperture and an aperture-to-isocenter distance of 30 cm, the H/D values measured at the isocenter were approximately 0.3 mSv/Gy for all snouts with a 100-MeV beam. The H/D values increased to 10.7, 14.5, and 15.1 mSv/Gy, respectively, for small, medium, and large snouts when the beam energy increased to 250 MeV. At the out-of-field location, H/D values increased from 0.1 to 2.7, 3.0, and 3.2 mSv/Gy, respectively, for small, medium, and large snouts. When the aperture-to-isocenter distance was changed from 10 to 40 cm, the H/D value at the isocenter dropped 70%. The H/D value doubled for the modulated beam relative to the nonmodulated beam. Open apertures reduced the neutrons produced in the nozzle, but increased those produced in the phantom.
  
  
  CONCLUSIONS
  Our data showed that changes in the four factors studied affect the H/D value in predictable ways which permits an estimate of a patient's neutron exposure.
authors.affiliation: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, TX 77030, USA.
authors.forename: Xin
authors.initials: X
authors.lastname: Wang
authors.forename: Narayan
authors.initials: N
authors.lastname: Sahoo
authors.forename: Ronald X
authors.initials: RX
authors.lastname: Zhu
authors.forename: John R
authors.initials: JR
authors.lastname: Zullo
authors.forename: Michael T
authors.initials: MT
authors.lastname: Gillin
mesh_terms: D000465:Algorithms; D001932:Brain Neoplasms / Q000532:radiotherapy; D002138:Calibration; D019471:Elementary Particle Interactions; D006801:Humans; D008297:Male; D009502:Neutrons / Q000627:therapeutic use; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D061766:Proton Therapy; D011522:Protons; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D012062:Relative Biological Effectiveness; D012542:Scattering, Radiation; D013125:Spinal Neoplasms / Q000532:radiotherapy; D013679:Technology, Radiologic / Q000295:instrumentation
chemical_list: D011522:Protons