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

Purpose and background: A feasibility study is performed to evaluate the po ssibility of using the transit dose of portal images on the beam axis to me asure the accuracy in dose delivery. The algorithm and the method are teste d on a breast phantom and on patients with a breast disease. Materials and methods: To estimate the transit dose at various air gaps beh ind the patient, a method is proposed which applies, for a given air gap, t he inverse square law to the primary component of the exit dose and an expe rimentally 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 fun ction for the variation of the scattered component with the air gap, determ ined by phantom measurements, is modelled by an analytical function which c ontains only field size, air gap and one energy-dependent parameter. Results: The measurements on the breast phantom yield a maximum deviation b etween 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 m easurements 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 est imated transit dose. The standard deviation of the distribution is 4.4%. Th e method seems to be very sensitive to patient positioning and to discrepan cies in breast thicknesses used for treatment planning. Conclusion: Preliminary results on breast patients show that the method pro posed to evaluate transit doses on the beam axis from portal images may be a valuable alternative to conventional in vivo exit dosimetry. The method c an be implemented in a simple way and does not require additional time duri ng the irradiation session, as exit dosimetry with diodes does. The transit dose is only considered in one point. Nevertheless, in the framework: of q uality assurance of treatment delivery, this study is an example of the pos sibilities of monitoring at the same time the visual evaluation of the irra diated volume as well as the dosimetric control (i.e. in Gy) of treatment d elivery with portal images. (C) 2000 Elsevier Science Ireland Ltd. All righ ts reserved.