Dose-Response Function of an Amorphous Silicon Electronic Portal Imaging Device

Electronic Portal Imaging Devices (EPID) are not only suited for patient set-up verification and detection of organ motion but can also be used for dosimetric verification of treatment techniques or in-vivo dosimetry. The aim of this work is to develop a mapping function which describes the relationship between the pixel value and exit dose of an amorphous silicon (a-Si) EPID. The dose-response function depends on field size, source-detector distance (SDD) and the monitor unit (MU) setting. To experimentally derive the coefficients, EPID data and ionization chamber in a mini-phantom, which are used as reference data, were fit and compared on the central axis for detector distances between 105cm to 125cm from the source, radiation field sizes from 5cm×5cm to 15cm×15cm and monitor unit settings of 50,100,150. All measurements were done using 6 MV photon beams at a Varian Clinac 23EX medical accelerator equipped with the Varian aS1000 a-Si EPID system. Each dose profile was evaluated by comparing the EPID with the ionization chamber measurement. The comparisons indicate a maximum deviation was <; 6%. It is found that accuracies of better than 1% could be obtained if field size ≥10cm×10cm, because the effect of the lateral scatter was bigger in small field size. It is concluded that, using our model, EPID measurements can be used to estimate accurate two-dimensional dose distribution in a 6MV beam with 300MU/min.