Evaluation of the twin chamber signal ratio, a physical estimate of the low- energy photon contribution to dose within radiotherapy fields

Abstract

This investigation deals with an experimental method of identifying the presence of a low-energy component in the total photon spectrum inside an absorber irradiated by a high-energy photon beam. This is the so-called "twinchamber" method, in which the ratio of the signals produced by two air-filled ionisation chambers of approximately the same size, but one with a copper wall, the other with a graphited PMMA wall, is used as an indicator of the presence of low energy photons. This ratio is increased due to an excess of photoelectrons set in motion when the copper wall is hit by low-energy photons. We evaluate the applicability of a theoretical model of the signal ratio, which comprises the main factors determining the secondary electron fluence at the boundary surface of the chamber wall and the air filling. The variation of the signal ratio measured at various points inside a water or water-equivalent absorber irradiated by 6 MV photons has been compared with this theoretical model, and a close correlation has been found. The signal ratio is a very sensitive indicator of the spectral contribution by low-energy photons. Measured values of the signal ratio can be used for complementing other methods of the assessment of a lowenergy spectral component, e.g. Monte-Carlo-methods. Especially in critical regions of a radiotherapeutic photon field, such as the field borders and the regions outside the treatment field, measurements of the signal ratio may provide an easily available guidance to trace low-energy components of the photon spectrum. © 2009 Springer-Verlag.