Study On The Dose Response Characteristics Of An Electronic Portal Imaging Device And Its Applications In Dose Verification

Purpose Firstly, to investigate the dose response characteristics and its influenced factors of a scanning liquid ion-chambe electronic portal imaging device (EPID). Secondly, to use the EPID for dose verification by comparing the predicted portal dose images (PDIs) calculated with modified Pasma's algorithm, with PDIs measured with the EPID.Materials and Methods The study had two parts: the dose reponse and dose verification. In the first part, all experiments were performed on a Varian 600C/D accelerator(6MV x-ray) equipped with a Varian Portal Vision^TM MK2 type EPID. The dose response curves of the EPID were established by relating the pixel value output of the EPID with the incident radiation intensity received by the detector. And the influence factors such as gantry angle, field size and acquisition mode on the dose response curves were investigated through obtaining a set of dose response curves for different gantry angle, field size and acquisition mode. In the second part, through a set of on-axis and off-axis transmission measurements, all the parameters used in the PDI prediction algorithm were determined, and the dose verification method was tested with a homogeneous phantom and an inhomogeneous phantom. Two software tools, PIA and PDIExpect, were developed in the Visual C++ integrated environment to assist the study in the first and second part respectively. Results The relationship between dose rates and pixel values was not linear, so all the characteristic curves had been fitted by using polynomial functions. The dose response is strongly influenced by image acquisition mode including pulse repitition frequency and EPID sampling mode. It slightly varies with gantry angle position to the points away from the central axis but kept constant on the central axis. The dose response curves were found to be independent of radiation field sizes. The results of the portal dose measurements with the EPID in two parts study showed that the dose values determined by the EPID were consistent with expected values within the discrepancies of 1%⁵% . The results of dose...