Development of an Accurate Monte Carlo Model of an Electronic Portal Imaging Device (EPID) Toward an Improved Epid Calibration Methodology
Abstract
Purpose
To design and validate a Monte Carlo electronic portal imaging device (EPID) model to reduce the uncertainty in the calibration of EPIDs for in-vivo dosimetry of external beam treatments by consolidating correction factors into a single direct measurement.
Methods
A detailed Monte Carlo model of an EPID was developed through disassembly of a decommissioned PerkinElmer XRD-1640 a-Si array detector (Elekta iViewGT), allowing physical dimensions of the 19 individual detector layers to be measured and explicitly modeled. Layer composition and substructure were informed by these measurements, together with manufacturer specifications and prior EPID modeling studies in the published literature. The EPID model was implemented in DOSXYZnrc and coupled to a tuned 6MV BEAMnrc model of an Elekta Synergy linear accelerator. Beam tuning was performed using in-water dose profile measurements acquired with a PTW microDiamond detector. First-stage validation of the EPID model was carried out by irradiating EBT4 films on the upstream and downstream surfaces of the EPID assembly. Measured and simulated ratios of upstream-to-downstream dose determined with film were compared, directly probing the accuracy of the beam and EPID models.
Results
Initial validation of the EPID model showed agreement between measured and simulated upstream-to-downstream dose ratios, with a passing rate of 93.3% at 2%/2mm for the dose ratio comparisons within the in-field region for a 10x10cm2 field. These results indicate that the model captures the dominant effects introduced by the detector, resulting in consistent modification of the beam due to the EPID in both measurements and simulations.
Conclusion
This work developed an EPID Monte Carlo model that has been shown to reproduce measured beam perturbations introduced by the detector. These findings support the validity of the physical representation of the EPID geometry and materials and provide a basis for further investigation of EPID-induced beam effects under varied irradiation conditions.