9 Validation of Elekta iViewGT A-Si EPID Model for Pre-Treatment Dose Verification of IMRT Fields

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. The EPID images were acquired using frame-averaging acquisition and were processed using in-house application software. The agreement between the predicted and measured dose distribution was analyzed using the gamma analysis technique with the acceptance criteria of 3%, 3 mm. The results show that the predicted dose distributions of four clinical IMRT treatment plans have a good agreement with the measurement. Three prostate IMRT plans have the average gamma pass rate more than 95% and a spinal IMRT plan has the average gamma pass rate of 94.27%. Overall, Monte Carlo simulation shows a suitability to be implemented for pre-treatment dose verification, promising a better accuracy than the existing algorithms in Treatment Planning System (TPS).