title: Linear accelerator output variations and their consequences for megavoltage imaging.
abstract: An experimental study of radiation output intensity fluctuations of a Philips SL25 linear accelerator is presented. Measurements are obtained using an electronic portal imaging device, and the consequences of the measured fluctuations for various different applications of megavoltage imaging including portal imaging, transit dosimetry and megavoltage computed tomography (MVCT) are discussed with examples. Fluctuations in output of +/- 0.7% (1 SD) are seen on every radiation pulse after photon noise and uncertainties caused by the detection system have been accounted for. Large fluctuations are also seen during the initial beam stabilization period (15%), during normal accelerator operation after the beam has been on for more than 1 min (4.5%) and during are therapy as a repeatable function of gantry angle (9%). Such output intensity fluctuations are shown to produce image artifacts in portal imaging devices with scanned detector readout and can also produce systematic errors in detector calibration that would lead to uncertainty in transit dose calculations. The propagation of these intensity fluctuations through MVCT image reconstruction is shown to produce ring artifacts in the reconstructed image. Sample portal and MVCT images are presented. All observed fluctuations in accelerator output are well within the manufacturer's specifications and do not affect the total dose delivered during normal treatment. Finally, megavoltage imaging is shown to be a powerful tool for accelerator quality assurance and treatment verification.
authors.affiliation: Joint Department of Physics, Institute of Cancer Research, Royal Marsden NHS Trust, Sutton, United Kingdom. mikep@icr.ac.uk
authors.forename: M
authors.initials: M
authors.lastname: Partridge
authors.forename: P M
authors.initials: PM
authors.lastname: Evans
authors.forename: M A
authors.initials: MA
authors.lastname: Mosleh-Shirazi
mesh_terms: D002138:Calibration; D004867:Equipment Design; D006801:Humans; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D017785:Photons; D011878:Radiotherapy / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D014057:Tomography, X-Ray Computed