A Monte Carlo model of Elekta iViewGT amorphous silicon electronic portal imaging device (a-Si EPID) has been validated for pre-treatment dose verification of clinical IMRT treatment plans. The simulation employed BEAMnrc and DOSXYZnrc MC codes to predict the dose deposition in the iViewGT a-Si EPID model. The predicted dose distributions were compared to the measured dose distributions obtained from the experiment. The measured EPID images were obtained by delivering a photon beam from Elekta Precise linac to the Elekta iViewGT a-Si EPID. The a-Si EPID was used as non-transit dosimetry with no extra build-up material placed above the EPID detector....