AbstractID: 8209 Title: Clinical Implementation of Electron Beam Verification with an
a-Si EPID
The bremsstrahlung contamination can be used for patient setup verification using an a-Si
flat panel Electronic Portal Imaging Device (EPID) during cancer treatments with
electron beams. In this work, we investigate the impact of the acquisition conditions on
the ability to use the image clinically for verification purposes. The principal parameters
considered and their range are; the electronic acquisition modes, free running or beam
pulse triggered, the integration time values, from 285 ms to 10 s, the beam energies from
6 MeV to 21 MeV and the integrated dose from 10 to 50 monitor units (M.U.). For each
combination of parameters, an open field image is acquired and used for detector gain
correction. In the case of electron beams, the gain image also compensates for the strong
angular dependence of the bremsstrahlung. Images of a quality control phantom were
acquired and processed to determine the modulation transfer function value. Three radioopaque markers of different sizes were inserted in a rando head phantom. Careful study
shows that the markers are visible at more energies when ITV of 1600 ms is used. With
increasing beam energies from 6 to 21 MeV, the bremsstrahlung intensity increases while
contrast decreases. This results in the best image quality obtained around a beam energy
of 15 MeV.