title: Calculations of absorbed fractions in small water spheres for low-energy monoenergetic electrons and the Auger-emitting radionuclides (123)Ι and (125)Ι.
abstract: OBJECTIVE
  To calculate the absorbed fraction (AF) of low energy electrons in small tissue-equivalent spherical volumes by Monte Carlo (MC) track structure simulation and assess the influence of phase (liquid water versus density-scaled water vapor) and of the continuous-slowing-down approximation (CSDA) used in semi-analytic calculations.
  
  
  METHODS
  An event-by-event MC code simulating the transport of electrons in both the vapor and liquid phase of water using appropriate electron-water interaction cross sections was used to quantify the energy deposition of low-energy electrons in spherical volumes. Semi-analytic calculations within the CSDA using a convolution integral of the Howell range-energy expressions are also presented for comparison.
  
  
  RESULTS
  The AF for spherical volumes of radii from 10-1000 nm are presented for monoenergetic electrons over the energy range 100-10,000 eV and the two Auger-emitting radionuclides (125)I and (123)I. The MC calculated AF for the liquid phase are found to be smaller than those of the (density scaled) gas phase by up to 10-20% for the monoenergetic electrons and 10% for the two Auger-emitters. Differences between the liquid-phase MC results and the semi-analytic CSDA calculations are up to ∼ 55% for the monoenergetic electrons and up to ∼ 35% for the two Auger-emitters.
  
  
  CONCLUSIONS
  Condensed-phase effects in the inelastic interaction of low-energy electrons with water have a noticeable but relatively small impact on the AF for the energy range and target sizes examined. Depending on the electron energies, the semi-analytic approach may lead to sizeable errors for target sizes with linear dimensions below 1 micron.
authors.affiliation: Medical Physics Laboratory, University of Ioannina Medical School, Ioannina, Greece.
authors.forename: Christos
authors.initials: C
authors.lastname: Bousis
authors.forename: Dimitris
authors.initials: D
authors.lastname: Emfietzoglou
authors.forename: Hooshang
authors.initials: H
authors.lastname: Nikjoo
mesh_terms: D000042:Absorption; D032701:Biomimetics; D004583:Electrons; D007457:Iodine Radioisotopes; D009010:Monte Carlo Method; D055585:Physical Phenomena; D014867:Water
chemical_list: D007457:Iodine Radioisotopes; D014867:Water