Modeling electronic portal imager response for pre-treatment verification of IMRT fields

Abstract

Quality assurance (QA) of an intensity modulated radiation therapy (IMRT) plan is more complex than that of a conventional plan. To improve the efficiency of QA, electronic portal imaging devices (EPIDs) can be used. The major objective of this work was to model the EPID response using a commercial treatment planning system for the purpose of pre-treatment IMRT dose verification.                Images were acquired with a Varian amorphous silicon flat panel portal imager (aS500) directly irradiated with a 6MV photon beam from a Clinac 21EX linear accelerator (Varian Medical Systems, Palo Alto, CA). Portal images were acquired for a variety of rectangular fields from which profiles and relative output factors were extracted. A dedicated machine model was created using the physics tools of the Pinnacle^3® (Philips Medical Systems, Madison, WI) treatment planning system to model the data. Starting with the known photon spectrum and assuming an effective depth of 7cm, machine model parameters were adjusted to best fit measured profile and output factors. The machine parameters of a second model, which assumed a 0.8MeV mono-energetic photon spectrum and an effective depth in water of 3cm, were also optimized. The second EPID machine model was used to calculate planar dose maps of simple geometrical IMRT fields as well as a 9-field IMRT plan developed for clinical trials credentialing purposes.                The choice of energy and depth for an EPID machine model influenced the best achievable fit of the optimized machine model to the measured data. When both energy and depth were reduced by a significant amount, a better overall fit was achieved.  In either case, the secondary source size and strength could be adjusted to give reasonable agreement with measured data. The g evaluation method was used to compare planar dose maps calculated using the second EPID machine model with the EPID images of small IMRT fields.  In each case >95% of points fall within 3% of the maximum dose or 3mm distance-to-agreement. These results are slightly poorer than those obtained using an ion chamber array which confirms agreement to within 2% of the maximum dose or 2mm distance to agreement for all points within these fields.