title: Measurement of changes in linear accelerator photon energy through flatness variation using an ion chamber array.
abstract: OBJECTIVE
  To compare the use of flatness versus percent depth dose (PDD) for determining changes in photon beam energy for a megavoltage linear accelerator.
  
  
  METHODS
  Energy changes were accomplished by adjusting the bending magnet current by up to ± 15% in 5% increments away from the value used clinically. Two metrics for flatness, relative flatness in the central 80% of the field (Flat) and average maximum dose along the diagonals normalized by central axis dose (FDN), were measured using a commercially available planner ionization chamber array. PDD was measured in water at depths of 5 and 10 cm in 3 × 3 cm(2) and 10 × 10 cm(2) fields using a cylindrical chamber.
  
  
  RESULTS
  PDD was more sensitive to changes in energy when the beam energy was increased than when it was decreased. For the 18-MV beam in particular, PDD was not sensitive to energy reductions below the nominal energy. The value of Flat was found to be more sensitive to decreases in energy than to increases, with little sensitivity to energy increases above the nominal energy for 18-MV beams. FDN was the only metric that was found to be sensitive to both increases and reductions of energy for both the 6- and 18-MV beams.
  
  
  CONCLUSIONS
  Flatness based metrics were found to be more sensitive to energy changes than PDD, In particular, FDN was found to be the most sensitive metric to energy changes for photon beams of 6 and 18 MV. The ionization chamber array allows this metric to be conveniently measured as part of routine accelerator quality assurance.
authors.affiliation: Department of Radiation Physics, Unit 94, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77030, USA.
authors.forename: Song
authors.initials: S
authors.lastname: Gao
authors.forename: Peter A
authors.initials: PA
authors.lastname: Balter
authors.forename: Mark
authors.initials: M
authors.lastname: Rose
authors.forename: William E
authors.initials: WE
authors.lastname: Simon
mesh_terms: D004735:Energy Transfer; D004867:Equipment Design; D019544:Equipment Failure Analysis; D007477:Ions; D010315:Particle Accelerators; D017785:Photons; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation*
chemical_list: D007477:Ions