Performance Evaluation of the Siemens Electronic Portal Imaging Device for IMRT Plan Verification

A commercially available amorphous Silicon Electronic Portal imaging device (aSi-EPID) installed onto Siemens Oncor Linear accelerator was evaluated with the objective to be implemented for IMRT quality assurance. EPID properties were investigated; these include reproducibility, dependences between the relative EPID responses and field size, dose rate were studied for both photon beams. To determine the inherent build-up of the EPID, varied thickness of Solid-water was placed onto the detector surface and changes in EPID signals were investigated. EPID measurements were compared with ionization chamber measurements (type 30013) connected to a UNIDOS electrometer (PTW-Freiberg) in Water phantom. The use of EPID dose maps was tested by comparing it with TPS-calculated one for IMRT plan applying gamma criteria of 3%/3mm. The results demonstrated that the aSi-EPID signals were reproducible, and response to the applied MUs were linear up to 100 MUs, and then the response became stable for MUs higher than 200, detector should be calibrated in this range. The results showed a clear dependency on the field size and energy. The dosimetric properties measured in this work shows promise that the aSi-EPID can be used for IMRT verifications.