title: Detector dose response in megavoltage small photon beams. II. Pencil beam perturbation effects.
abstract: OBJECTIVE
  To quantify detector perturbation effects in megavoltage small photon fields and support the theoretical explanation on the nature of quality correction factors in these conditions.
  
  
  METHODS
  In this second paper, a modern approach to radiation dosimetry is defined for any detector and applied to small photon fields. Fano's theorem is adapted in the form of a cavity theory and applied in the context of nonstandard beams to express four main effects in the form of perturbation factors. The pencil-beam decomposition method is detailed and adapted to the calculation of perturbation factors and quality correction factors. The approach defines a perturbation function which, for a given field size or beam modulation, entirely determines these dosimetric factors. Monte Carlo calculations are performed in different cavity sizes for different detection materials, electron densities, and extracameral components.
  
  
  RESULTS
  Perturbation effects are detailed with calculated perturbation functions, showing the relative magnitude of the effects as well as the geometrical extent to which collimating or modulating the beam impacts the dosimetric factors. The existence of a perturbation zone around the detector cavity is demonstrated and the approach is discussed and linked to previous approaches in the literature to determine critical field sizes.
  
  
  CONCLUSIONS
  Monte Carlo simulations are valuable to describe pencil beam perturbation effects and detail the nature of dosimetric factors in megavoltage small photon fields. In practice, it is shown that dosimetric factors could be avoided if the field size remains larger than the detector perturbation zone. However, given a detector and beam quality, a full account for the detector geometry is necessary to determine critical field sizes.
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: 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 KingdomMedical Physics, EBG MedAustron GmbH, Wiener Neustadt A-2700, Austria.
authors.forename: Hugo
authors.initials: H
authors.lastname: Palmans
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
mesh_terms: D009010:Monte Carlo Method; D017785:Photons; D011874:Radiometry / Q000379:methods*