title: Measured Neutron Spectra and Dose Equivalents From a Mevion Single-Room, Passively Scattered Proton System Used for Craniospinal Irradiation.
abstract: OBJECTIVE
  To measure, in the setting of typical passively scattered proton craniospinal irradiation (CSI) treatment, the secondary neutron spectra, and use these spectra to calculate dose equivalents for both internal and external neutrons delivered via a Mevion single-room compact proton system.
  
  
  METHODS
  Secondary neutron spectra were measured using extended-range Bonner spheres for whole brain, upper spine, and lower spine proton fields. The detector used can discriminate neutrons over the entire range of the energy spectrum encountered in proton therapy. To separately assess internally and externally generated neutrons, each of the fields was delivered with and without a phantom. Average neutron energy, total neutron fluence, and ambient dose equivalent [H* (10)] were calculated for each spectrum. Neutron dose equivalents as a function of depth were estimated by applying published neutron depth-dose data to in-air H* (10) values.
  
  
  RESULTS
  For CSI fields, neutron spectra were similar, with a high-energy direct neutron peak, an evaporation peak, a thermal peak, and an intermediate continuum between the evaporation and thermal peaks. Neutrons in the evaporation peak made the largest contribution to dose equivalent. Internal neutrons had a very low to negligible contribution to dose equivalent compared with external neutrons, largely attributed to the measurement location being far outside the primary proton beam. Average energies ranged from 8.6 to 14.5 MeV, whereas fluences ranged from 6.91 × 10(6) to 1.04 × 10(7) n/cm(2)/Gy, and H* (10) ranged from 2.27 to 3.92 mSv/Gy.
  
  
  CONCLUSIONS
  For CSI treatments delivered with a Mevion single-gantry proton therapy system, we found measured neutron dose was consistent with dose equivalents reported for CSI with other proton beamlines.
authors.affiliation: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas. Electronic address: rhowell@mdanderson.org.
authors.forename: Rebecca M
authors.initials: RM
authors.lastname: Howell
authors.affiliation: Department of Nuclear Engineering, Idaho State University, Pocatello, Idaho.
authors.forename: Eric A
authors.initials: EA
authors.lastname: Burgett
authors.affiliation: Department of Nuclear Engineering, Idaho State University, Pocatello, Idaho.
authors.forename: Daniel
authors.initials: D
authors.lastname: Isaacs
authors.affiliation: Department of Radiation Oncology, Washington University, St. Louis, Missouri.
authors.forename: Samantha G
authors.initials: SG
authors.lastname: Price Hedrick
authors.affiliation: Department of Radiation Oncology, Washington University, St. Louis, Missouri.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Reilly
authors.affiliation: Department of Radiation Oncology, Washington University, St. Louis, Missouri.
authors.forename: Leith J
authors.initials: LJ
authors.lastname: Rankine
authors.affiliation: Department of Radiation Oncology, Washington University, St. Louis, Missouri.
authors.forename: Kevin K
authors.initials: KK
authors.lastname: Grantham
authors.affiliation: Department of Radiation Oncology, Washington University, St. Louis, Missouri.
authors.forename: Stephanie
authors.initials: S
authors.lastname: Perkins
authors.affiliation: Department of Radiation Oncology, Washington University, St. Louis, Missouri.
authors.forename: Eric E
authors.initials: EE
authors.lastname: Klein
mesh_terms: D001921:Brain / Q000528:radiation effects; D002173:Cancer Care Facilities; D061888:Craniospinal Irradiation / Q000379:methods*; D006801:Humans; D009502:Neutrons; D061766:Proton Therapy / Q000379:methods*; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D012542:Scattering, Radiation; D013131:Spine / Q000528:radiation effects