title: Subcellular S-factors for low-energy electrons: a comparison of Monte Carlo simulations and continuous-slowing-down calculations.
abstract: OBJECTIVE
  To study the energy deposition by low-energy electrons in submicron tissue-equivalent targets by comparing two widely used methodologies, namely, the continuous-slowing-down-approximation (CSDA) convolution integral and the Monte Carlo (MC) simulation.
  
  
  METHODS
  An MC track-structure code that simulates collision-by-collision the complete slowing down process is used to calculate the energy deposition in spherical volumes of unit density water medium. Comparisons are made with calculations based on the CSDA convolution integral using both empirical and MC-based range-energy analytic formulae.
  
  
  RESULTS
  We present self-irradiation absorbed fractions and S-factors for monoenergetic electrons of initial energies from 0.1-10 keV distributed uniformly in spheres of 5, 10, 50, 100, 500, and 1000 nm radius. The MC and CSDA results were found, in some cases, to differ by a factor of 2 or more; differences generally increase with decreasing sphere size. Contrary to high energies, the uncertainties associated with the straight-ahead approximation implicit in the CSDA calculations are of the same order as those related to straggling and delta-ray effects.
  
  
  CONCLUSIONS
  The use of the CSDA methodology may be unsuitable for the sub-micron scale where a more realistic description of electron transport becomes important.
authors.affiliation: Medical Physics Laboratory, University of Ioannina Medical School, Ioannina, Greece.
authors.forename: D
authors.initials: D
authors.lastname: Emfietzoglou
authors.forename: K
authors.initials: K
authors.lastname: Kostarelos
authors.forename: P
authors.initials: P
authors.lastname: Hadjidoukas
authors.forename: C
authors.initials: C
authors.lastname: Bousis
authors.forename: A
authors.initials: A
authors.lastname: Fotopoulos
authors.forename: A
authors.initials: A
authors.lastname: Pathak
authors.forename: H
authors.initials: H
authors.lastname: Nikjoo
mesh_terms: D003198:Computer Simulation; D004583:Electrons; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011868:Radioisotopes; D011874:Radiometry; D012542:Scattering, Radiation; D013269:Stochastic Processes
chemical_list: D011868:Radioisotopes