title: Detector dose response in megavoltage small photon beams. I. Theoretical concepts.
abstract: OBJECTIVE
  To explain the reasons for significant quality correction factors in megavoltage small photon fields and clarify the underlying concepts relevant to dosimetry under such conditions.
  
  
  METHODS
  The validity of cavity theory and the requirement of charged particle equilibrium (CPE) are addressed from a theoretical point of view in the context of nonstandard beams. Perturbation effects are described into four main subeffects, explaining their nature and pointing out their relative importance in small photon fields.
  
  
  RESULTS
  It is demonstrated that the failure to meet classical cavity theory requirements, such as CPE, is not the reason for significant quality correction factors. On the contrary, it is shown that the lack of CPE alone cannot explain these corrections and that what matters most, apart from volume averaging effects, is the relationship between the lack of CPE in the small field itself and the density of the detector cavity. The density perturbation effect is explained based on Fano's theorem, describing the compensating effect of two main contributions to cavity absorbed dose. Using the same approach, perturbation effects arising from the difference in atomic properties of the cavity medium and the presence of extracameral components are explained. Volume averaging effects are also discussed in detail.
  
  
  CONCLUSIONS
  Quality correction factors of small megavoltage photon fields are mainly due to differences in electron density between water and the detector medium and to volume averaging over the detector cavity. Other effects, such as the presence of extracameral components and differences in atomic properties of the detection medium with respect to water, can also play an accentuated role in small photon fields compared to standard beams.
authors.affiliation: Acoustics and Ionising Radiation Team, National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom.
authors.forename: Hugo
authors.initials: H
authors.lastname: Bouchard
authors.affiliation: Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada.
authors.forename: Jan
authors.initials: J
authors.lastname: Seuntjens
authors.affiliation: Acoustics and Ionising Radiation Team, National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom.
authors.forename: Simon
authors.initials: S
authors.lastname: Duane
authors.affiliation: Centre hospitalier de l'Université de Montréal (CHUM), 1560 Sherbrooke est, Montréal, Québec H2L 4M1, Canada.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Kamio
authors.affiliation: Acoustics and Ionising Radiation Team, National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom Medical Physics, EBG MedAustron GmbH, A-2700 Wiener Neustadt, Austria.
authors.forename: Hugo
authors.initials: H
authors.lastname: Palmans
mesh_terms: D009010:Monte Carlo Method; D017785:Photons; D011874:Radiometry / Q000379:methods*