A method to estimate the transit dose on the beam axis for verification of dose delivery with portal images

Abstract

Purpose and background: A feasibility study is performed to evaluate the possibility of using the transit dose of portal images on the beam axis to measure the accuracy in dose delivery. The algorithm and the method are tested on a breast phantom and on patients with a breast disease. Materials and methods: To estimate the transit dose at various air gaps behind the patient, a method is proposed which applies, for a given air gap, the inverse square law to the primary component of the exit dose and an experimentally determined function for the scatter component of the exit dose. It is assumed that the primary component and the scattered component of the exit dose are given by the treatment planning system. The experimental function for the variation of the scattered component with the air gap, determined by phantom measurements, is modelled by an analytical function which contains only field size, air gap and one energy-dependent parameter. Results: The measurements on the breast phantom yield a maximum deviation between measured and estimated transit doses of 4.5%. The mean deviation is 0.9% with a standard deviation of the distribution of 2.3%. In vivo diode measurements on the same phantom yield a maximum deviation of 2.7%.Transit dose measurements on the beam axis for 45 portal images of breast patients show a mean deviation of 0.0% between the measured transit dose and the estimated transit dose. The standard deviation of the distribution is 4.4%. The method seems to be very sensitive to patient positioning and to discrepancies in breast thicknesses used for treatment planning. Conclusion: Preliminary results on breast patients show that the method proposed to evaluate transit doses on the beam axis from portal images may be a valuable alternative to conventional in vivo exit dosimetry. The method can be implemented in a simple way and does not require additional time during the irradiation session, as exit dosimetry with diodes does. The transit dose is only considered in one point. Nevertheless, in the framework of quality assurance of treatment delivery, this study is an example of the possibilities of monitoring at the same time the visual evaluation of the irradiated volume as well as the dosimetric control (i.e. in Gy) of treatment delivery with portal images.