Comment On 'Lateral Response Heterogeneity Of Bragg Peak Ionization Chambers For Narrow-Beam Photon And Proton Dosimetry'

Kuess et al (2017 Phys. Med. Biol. 62 206-27) irradiated several PTW 34070 ionization chambers with a narrow x-ray beam impinging at different positions of the detector entrance window and they observed that the reading of the ionization chambers decreased as the impact point of the beam approached the edge of the sensitive volume. They concluded that the radial response of the detector decreased with increasing radius and they proposed a correction factor to correct for that effect. This work shows, by means of a simple Monte Carlo simulation, that the conclusions of Kuess and co-workers do not seem to be supported by their experimental findings-quite the opposite, their experimental results seem to be compatible with a rather homogeneous radial response of the PTW 34070. It is shown that the radial decrease in the ionization chamber reading (as the impact point of the beam approaches the edge of the sensitive volume) is not due to a radial decrease of the response, but to the fact that part of the energy transferred to the secondary electrons is carried away and deposited outside the sensitive volume of the ionization chamber. As a consequence, it is believed that the method and correction factors proposed by Kuess and colleagues are not suitable to assess the response uniformity of large-area ionization chambers. Furthermore, the results of the publications that have used them thus far should be thoroughly revised.