title: Technical Note: On the impact of the incident electron beam energy on the primary dose component of flattening filter free photon beams.
abstract: OBJECTIVE
  For commercially available linear accelerators (Linacs), the electron energies of flattening filter free (FFF) and flattened (FF) beams are either identical or the electron energy of the FFF beam is increased to match the percentage depth dose curve (PDD) of the FF beam (in reference geometry). This study focuses on the primary dose components of FFF beams for both kinds of settings, studied on the same Linac.
  
  
  METHODS
  The measurements were conducted on a VersaHD Linac (Elekta, Crawley, UK) for both FF and FFF beams with nominal energies of 6 and 10 MV. In the clinical setting of the VersaHD, the energy of FFFM (Matched) beams is set to match the PDDs of the FF beams. In contrast the incident electron beam of the FFFU beam was set to the same energy as for the FF beam. Half value layers (HVLs) and a dual parameter beam quality specifier (DPBQS) were determined.
  
  
  RESULTS
  For the 6 MV FFFM beam, HVL and DPBQS values were very similar compared to those of the 6 MV FF beam, while for the 10 MV FFFM and FF beams, only %dd(10)x and HVL values were comparable (differences below 1.5%). This shows that matching the PDD at one depth does not guarantee other beam quality dependent parameters to be matched. For FFFU beams, all investigated beam quality specifiers were significantly different compared to those for FF beams of the same nominal accelerator potential. The DPBQS of the 6 MV FF and FFFM beams was equal within the measurement uncertainty and was comparable to published data of a machine with similar TPR20,10 and %dd(10)x. In contrast to that, the DPBQS's two parameters of the 10 MV FFFM beam were substantially higher compared to those for the 10 MV FF beam.
  
  
  CONCLUSIONS
  PDD-matched FF and FFF beams of both nominal accelerator potentials were observed to have similar HVL values, indicating similarity of their primary dose components. Using the DPBQS revealed that the mean attenuation coefficient was found to be the same within the uncertainty of 0.8% for 6 MV FF and 6 MV FFFM beams, while for 10 MV beams, they differed by 6.4%. This shows that the DPBQS can provide a differentiation of photon beam characteristics that would remain hidden by the use of a single beam quality specifier, such as %dd(10)x or HVL.
authors.affiliation: Department of Radiation Oncology, Division Medical Physics, Medical University Vienna, Vienna 1090, Austria and Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Vienna 1090, Austria.
authors.forename: Peter
authors.initials: P
authors.lastname: Kuess
authors.affiliation: Department of Radiation Oncology, Division Medical Physics, Medical University Vienna, Vienna 1090, Austria and Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Vienna 1090, Austria.
authors.forename: Dietmar
authors.initials: D
authors.lastname: Georg
authors.affiliation: EBG MedAustron GmbH, Wiener Neustadt 2700, Austria and National Physical Laboratory, Teddington TW 11 0LW, United Kingdom.
authors.forename: Hugo
authors.initials: H
authors.lastname: Palmans
authors.affiliation: Department of Radiation Oncology, Division Medical Physics, Medical University Vienna, Vienna 1090, Austria and Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Vienna 1090, Austria.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lechner
mesh_terms: D000465:Algorithms; D004583:Electrons; D010315:Particle Accelerators; D017785:Photons; D011137:Polystyrenes; D011874:Radiometry; D011879:Radiotherapy Dosage; D012542:Scattering, Radiation
chemical_list: D011137:Polystyrenes