title: Monte Carlo-based investigations on the impact of removing the flattening filter on beam quality specifiers for photon beam dosimetry.
abstract: OBJECTIVE
  The impact of removing the flattening filter in clinical electron accelerators on the relationship between dosimetric quantities such as beam quality specifiers and the mean photon and electron energies of the photon radiation field was investigated by Monte Carlo simulations. The purpose of this work was to determine the uncertainties when using the well-known beam quality specifiers or energy-based beam specifiers as predictors of dosimetric photon field properties when removing the flattening filter.
  
  
  METHODS
  Monte Carlo simulations applying eight different linear accelerator head models with and without flattening filter were performed in order to generate realistic radiation sources and calculate field properties such as restricted mass collision stopping power ratios (L¯/ρ)airwater, mean photon and secondary electron energies. To study the impact of removing the flattening filter on the beam quality correction factors kQ, this factor for detailed ionization chamber models was calculated by Monte Carlo simulations. Stopping power ratios (L¯/ρ)airwater and kQvalues for different ionization chambers as a function of TPR1020 and %dd(10)xwere calculated. Moreover, mean photon energies in air and at the point of measurement in water as well as mean secondary electron energies at the point of measurement were calculated.
  
  
  RESULTS
  The results revealed that removing the flattening filter led to a change within 0.3% in the relationship between %dd(10)xand (L¯/ρ)airwater, whereby the relationship between TPR1020 and (L¯/ρ)airwater changed up to 0.8% for high energy photon beams. However, TPR1020 was a good predictor of (L¯/ρ)airwater for both types of linear accelerator with energies < 10 MeV with a maximal deviation between both types of accelerators of 0.23%. According to the results, the mean photon energy below the linear accelerators head as well as at the point of measurement may not be suitable as a predictor of (L¯/ρ)airwater and kQto merge the dosimetry of both linear accelerator types. It was possible to derive (L¯/ρ)airwater using the mean secondary electron energy at the point of measurement as a predictor with an accuracy of 0.17%. A bias between kQfor linear accelerators with and without flattening filter within 1.1% and 1.6% was observed for TPR1020 and %dd(10)xrespectively.
  
  
  CONCLUSIONS
  The results of this study have shown that removing the flattening filter led to a change in the relationship between the well-known beam quality specifiers and dosimetric quantities at the point of measurement, namely (L¯/ρ)airwater, mean photon and electron energy. Furthermore, the results show that a beam profile correction is important for dose measurements with large ionization chambers in flattening filter free beams.
authors.affiliation: Institute of Medical Physics and Radiation Protection, University of Applied Sciences Giessen, Wiesenstrasse 14, Giessen, D-35390, Germany.|University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany.
authors.forename: Damian
authors.initials: D
authors.lastname: Czarnecki
authors.affiliation: University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany.
authors.forename: Björn
authors.initials: B
authors.lastname: Poppe
authors.affiliation: Institute of Medical Physics and Radiation Protection, University of Applied Sciences Giessen, Giessen, D-35390, Germany.|Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Marburg, D-35043, Germany.|Frankfurt Institute for Advanced Studies (FIAS), Ruth-Moufang-Straße 1, 60438, Frankfurt am Main, Germany.
authors.forename: Klemens
authors.initials: K
authors.lastname: Zink
mesh_terms: D000465:Algorithms; D009010:Monte Carlo Method; D010315:Particle Accelerators; D017785:Photons; D011874:Radiometry; D011880:Radiotherapy Planning, Computer-Assisted
keywords: Monte Carlo; flattening filter free; photon dosimetry