SU-E-J-35: A Model for the Simulation of EPID Measurements in the BeamNRC Monte Carlo Package

The intent of this study was to create a virtual model of a simple Electronic Portal Image Device (EPID) in the BeamNRC Monte Carlo package which would simulate EPID measurements taken in vivo.

A typical accelerator was created in the BeamNRC software using published 6MV spectra from Mohan, et. al. and the basic head design of a Varian accelerator head. The particle fluence at the level of the mylar window was output into a planar 'phase space' file. The output of this accelerator was delivered to a phantom with a 1 mm thick slab of water acting as the detector array using the DosXYZ package. Additionally, the fluence from the same geometry was computed at the level of the virtualEPID (vEPID). The simulations were run for 2×2, 5×5, 10×10, 20×20, and 25×25 field sizes, as well as a simple IMRT field. The results of the open field simulations were then used for the parameterization of a deconvolution kernel using the form described by Renner, et. al.

The simulations were run with sufficient histories to produce a 1% uncertainty in the resulting fluence and dose matrices in the high dose/fluence region of the output. Additional software was created to convert 'phase space' files to fluence in the same grid as the vEPID. A final software package was written to iteratively determine the exponential fitting parameters described in Ritter, et. al. to deconvolve the detector function from the fluence measurements.

The system will allow one to simulate both EPID measurements, as well as remove the detector function from vEPIDsimulations for further use in fluence studies using BeamNRC. This will allow one to simulate a system for in vivo patient measurements using an EPID to study without the need for time on a medical accelerator, and without contributing additional radiation fatigue to the EPID device.