Evaluation of the two-voltage method for parallel-plate ionization chambers irradiated with pulsed beams

Abstract

Air-vented ionization chambers exposed to clinical radiation beams may suffer from recombination during the drift of the charge carriers towards the electrodes. Thus, dosimetry protocols recommend the use of a correction factor, usually denominated saturation factor ($k_{\rm sat}$), to correct the ionization chamber readout for the incomplete collection of charge. The two-voltage method is the recommended methodology for the calculation of the saturation factor, however, it is based on the early Boag model, which only takes into account the presence of positive and negative ions in the ionization chamber and does not account for the electric field screening or the free electron contribution to the signal. The numerical simulation shows a better agreement with the experimental data than the current analytical theories in terms of charge collection efficiency. The classical two-voltage method, systematically overestimates the saturation factor, with differences increasing with dose per pulse also present at low dose per pulse. These results may have implications for the dosimetry with ionization chambers in therapy modalities that use a dose per pulse higher than conventional radiotherapy such as intraoperative radiotherapy but also in conventional dose per pulse for ionization chambers that suffer from significant charge recombination.