Monte Carlo simulation of an aS500 EPID in direct detection configuration in 6 MV beams

Full text: The purpose of this study is to create a model for 6 MY photon beams from a linear accelerator, incident on an amorphous silicon electronic portal imaging device (EPID) in direct detection configuration. A Varian Clinac 21 EX linear accelerator was modelled using the BEAMnrc module of the EGSnrc Monte Carlo code. The phasespace file was benchmarked by comparison to the dose profiles measured by a diode detector scanning a water tank. Depth doses and dose profiles were compared for a 15 x 15 and a 40 x 40 cm2 field size. In order to convert the Varian aS500 EPID used in this study to direct detection configuration, the copper buildup and all layers above the detector were removed, except a black sheet of paper that blocks the ambient light. Modelling the direct EPID was carried out using the DOSXYZnrc module and the dose was scored in the glass layer. The EPID model was tested by comparison of a 15 x 15 cm2 direct EPlD image profile with the dose profile calculated by the simulation. The excellent agreement of the measured and calculated depth doses and dose profiles at various depths confirmed the accuracy of the accelerator head model. The EPID model agreed with the measurements within 1.4% in the in-field region. The differences between the EPlD model and measurements may be attributed to scoring the dose in the glass layer. The model seems to be reliable for further investigation of transit dosimetry using aS500 EPIDs. (author)