title: MCNPX simulation of proton dose distributions in a water phantom.
abstract: BACKGROUND
  This study presents the Monte Carlo N-Particles Transport Code, Extension (MCNPX) simulation of proton dose distributions in a water phantom.
  
  
  METHODS
  In this study, fluence and dose distributions from an incident proton pencil beam were calculated as a function of depth in a water phantom. Moreover, lateral dose distributions were also studied to understand the deviation among different MC simulations and the pencil beam algorithm. MCNPX codes were used to model the transport and interactions of particles in the water phantom using its built-in "repeated structures" feature. Mesh Tally was used in which the track lengths were distributed in a defined cell and then converted into doses and fluences. Two different scenarios were studied including a proton equilibrium case and a proton disequilibrium case.
  
  
  RESULTS
  For the proton equilibrium case, proton fluence and dose in depths beyond the Bragg peak were slightly perturbed by the choice of the simulated particle types. The dose from secondary particles was about three orders smaller, but its simulation consumed significant computing time. This suggests that the simulation of secondary particles may only be necessary for radiation safety issues for proton therapy. For the proton disequilibrium case, the impacts of different multiple Coulomb scattering (MCS) models were studied. Depth dose distributions of a 70 MeV proton pencil beam in a water phantom obtained from MCNPX, Geometry and Track, version 4, and the pencil beam algorithm showed significant deviations between each other, because of different MCS models used.
  
  
  CONCLUSIONS
  Careful modelling of MCS is necessary when proton disequilibrium exists.
authors.forename: Chung-Chi
authors.initials: CC
authors.lastname: Lee
authors.forename: Yun-Jing
authors.initials: YJ
authors.lastname: Lee
authors.forename: Shih-Kuan
authors.initials: SK
authors.lastname: Chen
authors.forename: Bing-Hao
authors.initials: BH
authors.lastname: Chiang
authors.forename: Chuan-Jong
authors.initials: CJ
authors.lastname: Tung
authors.affiliation: Department of Medical Imaging and Radiological Sciences, College of Medicine, Chang Gung University; Medical Physics Research Center, Institute for Radiological Research, Chang Gung Memorial Hospital at Linkou/Chang Gung University College of Medicine, Taoyuan, Taiwan.
authors.forename: Tsi-Chian
authors.initials: TC
authors.lastname: Chao
mesh_terms: D000465:Algorithms; D006801:Humans; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D061766:Proton Therapy / Q000379:methods; D014867:Water
chemical_list: D014867:Water