title: Measured electron energy and angular distributions from clinical accelerators.
abstract: Electron energy spectra and angular distributions, including angular spreads, were measured using magnetic spectrometer techniques, at isocenter, for two clinical linear accelerators: one scanning beam machine, which achieves field flatness by scanning a pencil beam over the desired field at the patient plane, and one scattering foil machine, which disperses the electrons through a graded-thickness scattering foil. All measurements were made at isocenter (in the patient plane), in air, 1 m from the nominal accelerator source. The energy measurements were confined to electrons traveling along the central axis; any widely scattered electrons were effectively neglected. The energy spectra of the scanning beam machine are all of nearly Gaussian shape and energy full-width-at-half-maximum intensity (FWHM) of about 5% of the peak mean energy (denoted (E0)*). The energy spectra of the scattering foil machine have a variety of forms as a function of energy, including even spectra with double peaks, and spectra which changed with time. The FWHM values ranged from 9%-22% of (E0)*. The angular spread measurements, at isocenter, yielded sigma theta (x) x (E0)* approximately 295 mrad-MeV for the scanning beam machine, and 346 mrad-MeV for the scattering foil machine, where sigma theta x denotes the standard deviation of the plane-projected angular distribution. These angular spreads are 30%-40% smaller than angular spreads reported by others on a very similar machine using the penumbra method. Possible causes of this discrepancy are discussed.
authors.affiliation: James Graham Brown Cancer Center, Department of Radiation Oncology, University of Louisville, Kentucky 40202, USA. jodeasy@roentgen.bcc.louisville.edu
authors.forename: J O
authors.initials: JO
authors.lastname: Deasy
authors.forename: P R
authors.initials: PR
authors.lastname: Almond
authors.forename: M T
authors.initials: MT
authors.lastname: McEllistrem
mesh_terms: D004583:Electrons; D006801:Humans; D008962:Models, Theoretical; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011878:Radiotherapy / Q000295:instrumentation* / Q000379:methods; D011879:Radiotherapy Dosage