title: A semi-empirical method for calculating the effect of air gap space on output factors on electron beam dose distributions.
abstract: BACKGROUND
  A simple approach to calculate the effect of air gap on output factors on electron beam dose distribution is presented.
  
  
  METHODS
  The method accounts for variations of pencil beam parameters using a model developed by Bruinvis et al. [4,5]. The evaluation of this method is based on measurements of the output factors at various distances between the final collimating device and the phantom surface.
  
  
  RESULTS
  Comparison of calculations and measurements of output factors for various cone sizes and 0, 2 and 4 cm air gaps show agreement to within approximately 1.5% for electron energies of 6-13.5 MeV and field sizes of 5.3-10 cm in diameter.
  
  
  CONCLUSIONS
  The accuracy of this semi-empirical method can be considered clinically acceptable and reduces the amount of experimental work needed.
authors.affiliation: Hôpital-clinique Claude Bernard, Service de radiotherapie, Metz, France.
authors.forename: H
authors.initials: H
authors.lastname: Kharrati
authors.forename: P
authors.initials: P
authors.lastname: Aletti
authors.forename: I A
authors.initials: IA
authors.lastname: Bruinvis
mesh_terms: D000388:Air; D004583:Electrons; D006801:Humans; D008962:Models, Theoretical; D015588:Observer Variation; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results