A method to determine the water kerma in a phantom for x-rays with energies up to 40 keV

Abstract

In brachytherapy, as in any form of radiation therapy, accurate dosimetry in terms of absorbed dose to water is required. The output of brachytherapy sources has, up to now, been specified as the reference air kerma rate at 1 m distance. For this reason, the Physikalisch-Technische Bundesanstalt is developing a primary standard to determine the absorbed dose to water at the point of measurement within a water phantom. A candidate for the standard to be newly established is a water-equivalent parallel-plate ionization chamber with continuously adjustable electrode spacing, forming an extrapolation chamber in a phantom of water-equivalent material.In this work, a method is introduced to determine the absorbed dose to water with an extrapolation chamber in a phantom of water-equivalent material. Thereby, a conversion factor C(xi+1, xi) is applied to the difference of the ionization charges measured at two different plate separations xi and xi+1. In contrast to already described methods, the applied method is based on fundamental dosimetric relations without the use of a wall-less chamber approximation. It will be shown that by using the wall-less chamber approximation, a term contributing to the absorbed dose to water is overlooked. This term is given by the net energy fluence of the secondary electrons at the surface of the measuring volume.The introduced method is verified experimentally by a comparison with water kerma values obtained from air kerma measurements free in air and applying a conversion factor to water kerma obtained by Monte Carlo (MC) calculations for the conditions of the experiment. Evidence is produced showing that the new model significantly increases the consistency of experimental and MC results.