EPID dosimetry – configuration and pre-treatment IMRT verification

The electronic portal imaging device (EPID) is used for patient setup during radiotherapy sessions. Dosimetric verification is done using ion chambers, diodes and thermoluminescence detectors. In intensity-modulated radiation therapy (IMRT) the dosimetry is a sophisticated and time-consuming task. The simultaneous use of amorphous silicon (aSi) detectors and the transit dosimetry (TD) option in the treatment planning system (TPS) (Eclipse, Varian) enables dosimetric pre-treatment verification of the IMRT plans. The purpose of this study was to calibrate the EPID and TPS and to evaluate the usefulness of that method for dose verification in IMRT technique. The first step was calibration of the aSi EPID mounted on three linear accelerators (Clianc23EXS, Varian). Afterwards, configuration of the calculation algorithm in TPS was carried out. Then dosimetric characteristics of the EPID were investigated. The EPID response depending on the beam mode, treatment time and static square field size was measured. The same measurements were repeated twice for three accelerators and analyzed. Additionally, three IMRT plans were treated for the pre-treatment dose evaluation. The calculated dose matrix was compared with the delivered one. The similarity of the calculated and measured fluency maps was evaluated by means of gamma index and score factor in Eclipse. The linearity of the EPID signal was proven. For both beam modes EPID response is proportional to treatment time and field size, within the considered field size range. The gamma evaluation indicates good correlation between predicted and acquired EPID image, although some differences in a high gradient area were found. We found the EPID-based pre-treatment IMRT verification method to be a good quality assurance (QA) procedure. Quite frequent control of the method and periodic recalibration of the used device are required.