title: Study of commercial detector responses in non-equilibrium small photon fields of a 1000MU/min CyberKnife system.
abstract: OBJECTIVE
  The purpose of this study was to analyze the detector responses in non-equilibrium small photon fields.
  
  
  METHODS
  Five detectors (PTW 31014 ionization chamber, PTW 60016, PTW 60017 and Sun Nuclear EDGE diodes and PTW 60003 diamond detector) and one passive dosimeter (Harshaw micro-LiF) as well as a 1000MU/min CyberKnife were modeled with the PENELOPE Monte Carlo code. Field factors, [Formula: see text] were calculated and perturbations due to volume averaging effect, active material effect and coating effect were quantified for the five detectors and passive dosimeter.
  
  
  RESULTS
  The PTW 31014 ionization chamber under-response is mainly due to the fluence perturbation caused by the presence of air as detecting material. Regarding diodes, the high density materials used in their active volume and in their coating is responsible for their over-response. Regarding the PTW 60003 diamond, its under-response for the 5mm field size is due to a large volume averaging effect whereas for largest field sizes a nearly perfect compensation between the volume averaging effect and the material effect due to the diamond density occurs. Despite its small size, a volume averaging effect was observed for the micro-LiF for the 5mm field size.
  
  
  CONCLUSIONS
  Perturbations due to volume averaging effect, active material effect and coating effect were investigated and quantified for five active detectors. Since these perturbations can cause opposite effects, wrong conclusions may be drawn regarding the radiological water-equivalence of detectors. Thus, we recommend performing such a study for each novel detector available on the market.
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Service de Dosimétrie Externe, Laboratoire de Dosimétrie des Rayonnements Ionisants, Fontenay-aux-Roses, France. Electronic address: christelle.huet@irsn.fr.
authors.forename: Christelle
authors.initials: C
authors.lastname: Huet
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Service de Dosimétrie Externe, Laboratoire de Dosimétrie des Rayonnements Ionisants, Fontenay-aux-Roses, France.
authors.forename: Cyril
authors.initials: C
authors.lastname: Moignier
authors.affiliation: Service de Radiophysique, Centre François Baclesse, Caen, France.
authors.forename: Victor
authors.initials: V
authors.lastname: Barraux
authors.affiliation: Service de Radiophysique, Centre François Baclesse, Caen, France.
authors.forename: Cédric
authors.initials: C
authors.lastname: Loiseau
authors.affiliation: Service de Radiophysique, Centre François Baclesse, Caen, France.
authors.forename: Karine
authors.initials: K
authors.lastname: Sebe-Mercier
authors.affiliation: Service de Radiophysique, Centre François Baclesse, Caen, France.
authors.forename: Alain
authors.initials: A
authors.lastname: Batalla
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Service de Dosimétrie Externe, Laboratoire de Dosimétrie des Rayonnements Ionisants, Fontenay-aux-Roses, France.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Clairand
mesh_terms: D009010:Monte Carlo Method; D010315:Particle Accelerators; D017785:Photons; D011874:Radiometry / Q000295:instrumentation*; D016634:Radiosurgery
keywords: Detector response; Monte Carlo simulation; Small field dosimetry