title: Quality Assurance of a 50-kV Radiotherapy Unit Using EBT3 GafChromic Film: A Feasibility Study.
abstract: OBJECTIVE
  Radiochromic EBT3 film is gaining acceptance as a valuable dosimetry system for high-energy photon beams. The advantages of these films over other dosimetry systems are low spectral sensitivity and high spatial resolution. The aim of this study was to validate EBT3 film as a dosimeter for machine and treatment quality assurance (QA) of a 50-kV radiotherapy unit.
  
  
  METHODS
  Absolute and relative doses were acquired using EBT3 GafChromic films and compared to a parallel-plate ionization chamber (IC), the standard IC for low-energy X-rays. EBT3 was also used to evaluate beam profiles and output factors. Two films above each other, mimicking the clinical situation of a dosimeter on top of the skin, were simultaneously irradiated to evaluate EBT3 as in vivo dosimeter. All films were irradiated for 3 minutes, which corresponds with a surface dose of 3.25 ± 0.07 Gy.
  
  
  RESULTS
  A fifth-order polynomial function was found to be the best fit for the calibration curves. Good agreement between IC and EBT3 was found for absolute (0.92% for green and red color channels) and relative (1.2% and 1.0% for green and red color channels, respectively) dosimetry. Output factors for IC and EBT3 were comparable within 2.04% and 1.02% for the green and red color channels, respectively. Flatness and symmetry at the surface were within 2%. By applying film as in vivo dosimeter, an absorption of 4.70% needs to be taken into account with respect to the surface dose.
  
  
  CONCLUSIONS
  EBT3 GafChromic film is a feasible and valuable QA and dosimetry tool for a 50-kV radiotherapy unit. EBT3 can be used for absolute and relative dosimetry, measurement of output factors and beam profiles. In vivo patient-specific QA can also be performed if one corrects for the dose absorption of the film.
authors.affiliation: Department of Radiotherapy, UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Femke
authors.initials: F
authors.lastname: Steenbeke
authors.affiliation: Department of Radiotherapy, UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium thierry.gevaert@uzbrussel.be.
authors.forename: Thierry
authors.initials: T
authors.lastname: Gevaert
authors.affiliation: Department of Radiotherapy, UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Koen
authors.initials: K
authors.lastname: Tournel
authors.affiliation: Department of Radiotherapy, UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Benedikt
authors.initials: B
authors.lastname: Engels
authors.affiliation: Department of Radiotherapy, UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Dirk
authors.initials: D
authors.lastname: Verellen
authors.affiliation: Department of Radiotherapy, UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Guy
authors.initials: G
authors.lastname: Storme
authors.affiliation: Department of Radiotherapy, UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium.
authors.forename: Mark
authors.initials: M
authors.lastname: De Ridder
mesh_terms: D002138:Calibration; D004307:Dose-Response Relationship, Radiation; D005240:Feasibility Studies; D006801:Humans; D060051:Occupational Injuries / Q000517:prevention & control*; D011785:Quality Assurance, Health Care; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000517:prevention & control*; D011874:Radiometry / Q000295:instrumentation
keywords: EBT3 GafChromic film; absolute and relative dosimetry; contact X-ray radiotherapy; in vivo dosimetry; low-energy dosimetry