title: Photon beam quality variations of a flattening filter free linear accelerator.
abstract: OBJECTIVE
  Recently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off-axis ray angle for unflattened beams. In addition, a comparison was made with the off-axis energy variation in flattened beams.
  
  
  METHODS
  Two Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in-house developed device that can be easily mounted on the gantry. At St. Luke's Hospital (Dublin, Ireland), measurements were performed at 6 MV in narrow beam geometry with the gantry tilted around 270 degrees with pinhole collimators, an attenuator, and the chamber positioned on the table. All attenuation measurements were performed with ionization chambers and a buildup cap (2 mm brass) or a PMMA mini phantom (diameter 3 cm, measurement depth 2.5 cm).
  
  
  RESULTS
  For flattened 6 and 10 MV photon beams from the Elekta linac the relative HVL(theta) varies by about 11% for an off-axis ray angle theta = 10 degrees. These results agree within +/- 2% with a previously proposed generic off-axis energy correction. For unflattened beams, the variation was less than 5% in the whole range of off-axis ray angles up to 10 degrees. The difference in relative HVL data was less than 1% for unflattened beams at 6 and 10 MV.
  
  
  CONCLUSIONS
  Off-axis energy variation is rather small in unflattened beams and less than half the one for flattened beams. Thus, ignoring the effect of off-axis energy variation for dose calculations in unflattened beams can be clinically justified.
authors.affiliation: Department of Radiotherapy, Division Medical Radiation Physics, Medical University of Vienna, AKH Vienna, 1090 Vienna, Austria. dietmar.georg@akhwien.at
authors.forename: Dietmar
authors.initials: D
authors.lastname: Georg
authors.forename: Gabriele
authors.initials: G
authors.lastname: Kragl
authors.forename: Sacha af
authors.initials: Sa
authors.lastname: Wetterstedt
authors.forename: Patrick
authors.initials: P
authors.lastname: McCavana
authors.forename: Brendan
authors.initials: B
authors.lastname: McClean
authors.forename: Tommy
authors.initials: T
authors.lastname: Knöös
mesh_terms: D017076:Computer-Aided Design; D004867:Equipment Design; D019544:Equipment Failure Analysis; D010315:Particle Accelerators / Q000295:instrumentation*; D061766:Proton Therapy; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy / Q000295:instrumentation*; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity; D011522:Protons
chemical_list: D011522:Protons