title: Electron contamination in 4 MV and 10 MV radiotherapy x-ray beams.
abstract: A thin window parallel-plate ionization chamber was constructed for dose measurement in the build-up region of high energy radiotherapy photon beams. The chamber is an integral part of a perspex block. The entrance window is 12 microns Melinex foil with a thin aluminium surface. Cavity thickness is 1.45 mm. Surface doses for varying field sizes were found to increase almost linearly with the side length of a square field. The surface dose for a 10x10 cm 4 MV photon beam is 12.1% for an open field and this increases to 14.1% with a polycarbonate block tray in the beam. Similarly for a 10 MV photon beam the surface dose is 10.6% for an open field and this increases to 12.4% with a polycarbonate block tray. The difference between the dose for an open field and a field with a polycarbonate block tray inserted becomes more significant for larger field sizes. Electron contamination depth dose curves are determined for a 4 MV and 10 MV photon beam. This is achieved by subtracting a pure photon beam build-up curve generated by an EGS4 Monte Carlo simulation from the experimental build-up curve. The EGS4 curve is a theoretical, electron contamination free curve. The electron contamination curve (of the 10 MV photon beam) has depth dose characteristics similar to that of a broad low energy electron beam.
authors.affiliation: Physics Department, University of Waikato, Hamilton, New Zealand.
authors.forename: N R
authors.initials: NR
authors.lastname: Harper
authors.forename: P E
authors.initials: PE
authors.lastname: Metcalfe
authors.forename: P W
authors.initials: PW
authors.lastname: Hoban
authors.forename: W H
authors.initials: WH
authors.lastname: Round
mesh_terms: D004583:Electrons; D004867:Equipment Design; D004864:Equipment and Supplies; D009010:Monte Carlo Method; D011879:Radiotherapy Dosage / Q000592:standards*