PMID: 9167169May 1, 1997Paper

Off-axis primary-dose measurements using a mini-phantom

Medical Physics
S A Johnsson, C P Ceberg

Abstract

The characterization of the incident photon beam is usually divided into its dependence on collimator setting (head-scatter factor) and off-axis position (primary off-axis ratio). These parameters are normally measured "in air" with a build-up cap thick enough to generate full dose build-up at the depth of dose maximum. In order to prevent any influence from contaminating electrons, it has been recommended that head-scatter measurements are carried out using a mini-phantom rather than a conventional build-up cap. Due to the volume of the mini-phantom, the effects from attenuation and scatter are not negligible. In relative head-scatter measurements these effects cancel and the head scatter is thus a good representation of the variation of the incident photon beam with collimator setting. However, in off-axis measurements, attenuation and scatter conditions vary due to beam softening and do not cancel in the calculation of the primary off-axis ratio. The purpose of the present work was to estimate the effects from attenuation and phantom scatter in order to determine their influence on primary off-axis ratio measurements. We have characterized the off-axis beam-softening effect by means of narrow-beam transmission measurements t...Continue Reading

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Citations

Oct 26, 1999·Physics in Medicine and Biology·S A JohnssonP Nilsson
May 2, 2003·Medical Physics·Timothy C Zhu, Bengt E Bjärngard
Feb 24, 2010·Medical Physics·Dietmar GeorgTommy Knöös
Nov 1, 2002·Medical Physics·Mette S Thomsen, Niels Ulsø
May 8, 1999·Physics in Medicine and Biology·D Georg, A Dutreix

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