An intercomparison of 11 amorphous silicon EPIDs of the same type: implications for portal dosimetry

The use of electronic portal imaging devices (EPIDs) for portal dosimetry requires knowledge of their dosimetric properties. The pixel value response of amorphous silicon EPIDs of type Elekta iViewGT (TM) is known to be nonlinear with dose. However, it is not clear whether these nonlinearities vary with time and from one detector to another, respectively. In the present study, the dose response characteristics of 11 iViewGT EPIDs were investigated with respect to dose rate, total dose and field size. It was found that each detector needs to be individually calibrated, not only in terms of absolute sensitivity but also with respect to its relative response variations with exposure parameters. Doubling the dose rate typically increased the EPID signal between 1.4% and 2.8%. Changing the number of monitor units from 30 to 500 was accompanied by an increase in detector sensitivity between 1.7% and 2.8%. The EPID scatter factors were always within +/- 1%. It was observed that the dose-response behaviour was not stable with respect to time. Particularly within the first weeks of operation, detector ageing caused variations in both absolute sensitivity and relative response curves. It is recommended to establish a quality assurance programme if the amorphous silicon EPIDs are intended to be used for clinical portal dosimetry.