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 the ion chambers, diodes and thermoluminescence detectors. In Intensity Modulated Radiation Therapy (IMRT) the dosimetry is sophisticated and time consuming task. The simultaneous use of an amorphous silicon (aSi) detectors and Transit Dosimetry (TD) option in 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. 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. Furthermore, three IMRT plans were treated for the pretreatment dose evaluation. The calculated dose matrix was compared with delivered one. The similarity of the calculated and measured fluency maps were evaluated by means of gamma index in Eclipse. The linearity of the EPID signal was proven. For both beam modes EPIDs response is proportional to treatment time and field size, within the considered field size range. However, the obtained results can slightly differ for the different accelerators that are equipped with different imaging devices. The gamma evaluation indicates good correlation between predicted and acquired EPID image. Although some differences in a high gradient area were found. We found EPID based pre-treatment IMRT verification method as a good Quality Assurance (QA) procedure. Quite frequent control of the method and the periodic recalibration of the used device are required.