MEASURED ELECTRON-ENERGY AND ANGULAR-DISTRIBUTIONS FROM CLINICAL ACCELERATORS

Electron energy spectra and angular distributions, including angular s preads, were measured using magnetic spectrometer techniques, at isoce nter, for two clinical linear accelerators: one scanning beam machine, which achieves field flatness by scanning a pencil beam over the desi red field at the patient plane, and one scattering foil machine, which disperses the electrons through a graded-thickness scattering foil. A ll 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 sc attered electrons were effectively neglected. The energy spectra of th e scanning beam machine are all of nearly Gaussian shape and energy fu ll-width-at-half-maximum intensity (FWHM) of about 5% of the peak mean energy (denoted [E(0)]). The energy spectra of the scattering foil m achine have a variety of forms as a function of energy, including even spectra with double peaks, and spectra which changed with time. The F WHM values ranged from 9%-22% of [E(0)] The angular spread measuremen ts, at isocenter, yielded sigma(theta x) x [E(0)] approximate to 295 mrad-MeV for the scanning beam machine, and 346 mrad-MeV for the scatt ering foil machine, where sigma(theta x) denotes the standard deviatio n of the plane-projected angular distribution. These angular spreads a re 30%-40% smaller than angular spreads reported by others on a very s imilar machine using the penumbra method. Possible causes of this disc repancy are discussed. (C) 1996 American Association of Physicists in Medicine.