Using the iViewGT Electronic Portal Image for Quality Control of a Clinical-Use Radiation Beam Linear Accelerator

Abstract

The reference state of the iViewGT aSi EPID attached to our Elekta precise linac was established following the recommendation of AAPM. The panel was tested for both available energies: 6 MV and 15 MV. The resolution of the panel is evaluated as the Fourier transform of the edge spread function (ESF) of a 1,5 cm thick Pb block placed at the isocenter. Other simple artefacts placed in the beam are useful to determine the geometric precision and linearity, as well as contrast and signal-to-noise ratio. The results are not far from those stated by the supplier and measured by others for this type of detector. It was also established a QC protocol to verify the functionality of the Elekta acquisition software (iViewGT). The described procedures serve as guides for the acceptance and commissioning and the indicators, are used as reference for periodic QC of the EPID. The feasibility of using iViewGT for beam quality control was demonstrated. From an image (16 bits and 1024x1024 pixels) acquired by a short exposure we obtained symmetry, flatness, penumbra and field size. The results were validated against the system “Hermes”, used in the weekly quality control. The constancy of the beam quality is evaluated by placing a 5mm thick Pb block on the panel surface. An exposure acquired with metallic pointers aiming at the centre of the crosswire is useful to determine the coincidence of light/ radiation beam centre. We developed procedures to analyse and process the images using a combination of packages available in the web: ImageJ and DoseLab.