title: Extraction of depth-dependent perturbation factors for parallel-plate chambers in electron beams using a plastic scintillation detector.
abstract: OBJECTIVE
  This work presents the experimental extraction of the overall perturbation factor PQ in megavoltage electron beams for NACP-02 and Roos parallel-plate ionization chambers using a plastic scintillation detector (PSD).
  
  
  METHODS
  The authors used a single scanning PSD mounted on a high-precision scanning tank to measure depth-dose curves in 6, 12, and 18 MeV clinical electron beams. The authors also measured depth-dose curves using the NACP-02 and PTW Roos chambers.
  
  
  RESULTS
  The authors found that the perturbation factors for the NACP-02 and Roos chambers increased substantially with depth, especially for low-energy electron beams. The experimental results were in good agreement with the results of Monte Carlo simulations reported by other investigators. The authors also found that using an effective point of measurement (EPOM) placed inside the air cavity reduced the variation of perturbation factors with depth and that the optimal EPOM appears to be energy dependent.
  
  
  CONCLUSIONS
  A PSD can be used to experimentally extract perturbation factors for ionization chambers. The dosimetry protocol recommendations indicating that the point of measurement be placed on the inside face of the front window appear to be incorrect for parallel-plate chambers and result in errors in the R50 of approximately 0.4 mm at 6 MeV, 1.0 mm at 12 MeV, and 1.2 mm at 18 MeV.
authors.affiliation: Département de Radio-Oncologie, Centre hospitalier de l'Université de Montréal, Québec, Canada. frederic.lacroix.chum@ssss.gouv.qc.ca
authors.forename: Frédéric
authors.initials: F
authors.lastname: Lacroix
authors.forename: Mathieu
authors.initials: M
authors.lastname: Guillot
authors.forename: Malcolm
authors.initials: M
authors.lastname: McEwen
authors.forename: Claudiu
authors.initials: C
authors.lastname: Cojocaru
authors.forename: Luc
authors.initials: L
authors.lastname: Gingras
authors.forename: A Sam
authors.initials: AS
authors.lastname: Beddar
authors.forename: Luc
authors.initials: L
authors.lastname: Beaulieu
mesh_terms: D019544:Equipment Failure Analysis / Q000295:instrumentation; D010969:Plastics / Q000528:radiation effects*; D011874:Radiometry / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000295:instrumentation*; D015203:Reproducibility of Results; D012588:Scintillation Counting / Q000295:instrumentation*; D012680:Sensitivity and Specificity
chemical_list: D010969:Plastics