Further uses of electronic portal imaging

Full text: Electronic portal imaging (EPI) is a system designed for acquiring digital images in radiation therapy. The principal use of EPI is the analysis of set-up parameters, such as beam size, field alignment, and portal location with respect to anatomical structures. Dosimetric studies have also been undertaken determining the transmission dose. Quality assurance (QA) using an electronic portal imaging device (EPID) is a relatively new procedure being investigated. Previous investigations as a QA tool have largely concentrated on areas such as position of photon isocentre, light and radiation field congruence, and MLC leaf transmission and positional reproducibility. It is the idea of this project to use the EPID for electron beams. Current research into electrons for portal imaging deals only with the secondary photon contamination that is always present. It is our intention to show that quality assurance procedures can be performed on electron beams using an EPID. The Varian PortalVision is a matrix ion chamber EPID system attached to a Vanan Clinac 2100C. Electron energies used were 6, 9, 12, 15, and 18MeV. Electron beam parameters were measured using a water phantom. Properties were then investigated using PortalVision. Images were obtained using a range of absorber thicknesses to determine both percentage depth doses and depth dose profiles. Percentage depth doses were characterized using the EPID for each electron energy. Depth dose profiles were plotted and compared against phantom-measured profiles to determine the penumbral width. Effective depth of measurement of the EPID was also measured for each energy. Results have demonstrated the validity of using an EPID as a quality assurance tool for electrons, with little preparation required to prepare the system. Benefits of an EPID for electron QA include gathering data within a few short minutes, and that an EPID has the ability to conduct some measurements at any given gantry angle, whereas a water phantom is generally limited to working at the cardinal gantry angles. Copyright (2000) Australasian College of Physical Scientists and Engineers in Medicine