Pre-treatment verification of IMRT fields produced by an Elekta Synergy BM LINAC with an iView GT EPID and a commercial dosimetric software: validation of the SunNuclear EPIDose system.

Purpose: Electronic Portal Imaging Devices (EPID) potentially offer an attractive alternative to other detectors such as chamber or diode arrays for IMRT pre-treatment verifications, because of their high resolution and because there is no detector set-up time, so the measurement is less LINAC-time-consuming. The purpose of this work is to commission and test a commercial software for the conversion of EPID images to dose distributions at the isocenter at 5cm water equivalent depth. Methods and materials: The SunNuclear EPIDose system was commissioned for an Elekta Synergy Beam Modulator LINAC and an iView GT EPID. The first step was the acquisition of measurements with different field sizes and varying MUs, both with the MapCHECK diode array and with the EPID, without any phantom. The LINAC output was tuned to the reference value used by our TPS. Basing the EPIDose physics modelling on these measurements, the images acquired in future sessions with the EPID will be converted to absolute dose. In every patient plan pre-treatment verification session with EPIDose, a reference image has to be acquired. The images are acquired in IMRT mode, Single exposure, Frame averaging maximum. A DICOM image is saved for each IMRT segment. The EPIDose software corrects for the pixel frame factor and the automatic normalization. This work aims to test the overall dosimetric accuracy of the dose distributions resulting from the EPIDose system, which depends both on the intrinsic limits of the EPID and on the goodness of the commissioning and of the EPIDose physics modelling. Time lag effects and PRF dependence are not explicitly addressed by EPIDose model, and the possibility to neglect these effects has been tested in terms of overall contribution. In this study the model was optimized and tested on ten clinical patient plans. Results: The gamma passing rates in the comparison of the measured dose distributions with the calculated one are similar to those obtained with MapCHECK. Conclusion: EPIDose has proven to be a possible alternative to MapCHECK for pre-treatment verification of IMRT plans, provided that the physics modelling has been commissioned for the specific LINAC and EPID. The LINAC time necessary for data acquisition is reduced, but the data transmission and elaboration process has to be improved to become competitive respect to MapCHECK. Further optimization of the EPIDose procedure and investigation of PRF dependence and time lag effects could be appropriate.