title: Off-axis chamber response in the depth of photon dose maximum.
abstract: Measurements as well as Monte Carlo simulations are presented to investigate the deviation between the dose to water and the value measured by an ionization chamber. These deviations are evaluated at different depths (1.5 and 10 cm) and at an off-axis position of 15 cm. It is shown that an ionization chamber can produce a measuring signal, which is up to 2.5% too low, compared to the dose, when measurements are performed at shallow depths and far off-axis. The reason for this underresponse is found in the variation of the wall correction factor. As a result of the variation of the radiation spectra with depth and position the dose to the air volume, which originates from the wall, varies and therefore changes the wall correction factor.
authors.affiliation: Sektion für Biomedizinische Physik, Universitätsklinik für Radioonkologie Tübingen, Hoppe-Seyler Str 3, 72076 Tübingen, Germany. ordohm@med.uni-tuebingen.de
authors.forename: Oliver S
authors.initials: OS
authors.lastname: Dohm
authors.forename: Matthias
authors.initials: M
authors.lastname: Fippel
authors.forename: Gunter
authors.initials: G
authors.lastname: Christ
authors.forename: Fridtjof
authors.initials: F
authors.lastname: Nüsslin
mesh_terms: D002138:Calibration; D003198:Computer Simulation; D004867:Equipment Design; D019544:Equipment Failure Analysis; D008954:Models, Biological; D015233:Models, Statistical; D017785:Photons; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods*; D012062:Relative Biological Effectiveness