Simple Proposal for Dosimetry with an Elekta iViewGT™ Electronic Portal Imaging Device (EPID) Using Commercial Software Modules

Background: An electronic portal imaging device (EPID) is used to control for patient setup and positioning during fractionated radiotherapy. Due to the rising complexity and conformity of irradiation techniques, the demand for an accurate verification of the dose delivered to the patient has also increased. The purpose of this study was to investigate a simple guidance for dosimetry with an Elekta iViewGT (TM) EPID using commercial software modules. Material and Methods: EPID measurements were performed using an Elekta iViewGT (TM) EPID on a linear accelerator with 6 MV x-ray beam. The EPID signal was studied for reproducibility, as well as characteristics as a function of dose, dose rate, and field size. A series of experiments, comparing the response of the flat panel imager and ionization chamber measurements of dose, determine the parameters for the calibration model. EPID measurements were also compared with calculations of the treatment planning system. Results: We found a stable response of the EPID signal over a period of 14 months. It showed nonlinearity depending on dose up to 6.8%. There were low oscillations up to 1.2% depending on dose rate. For all fields, the calibrated flat panel profiles match the measured and calculated dose profiles with maximum deviation of 2-3% for the in-field region. In the high gradient areas, higher differences up to 6% were found. Conclusions: The gamma evaluation indicates good correlation between predicted and acquired EPID images. The EPID-based pretreatment IMRT verification method will help to improve the quality assurance procedure.