Abstract
to investigate the impact of low atomic number (Z) targets and detector design on megavoltage (MV) portal image contrast. two experimental beams were generated by replacing flattening filtration of a 2100EX linac with beryllium (Be) and aluminum (Al) targets and using the linac in 6 MeV electron mode. A standard MV contrast phantom was used to quantify planar image contrast for the standard 6MV and the 6MeVAl beam, incident on an amorphous silicon (a-Si) detector. Contrast versus separation for 6MV and 6MeV/Al was quantified using a 1 cm bone/solid water slab within increasing thicknesses of solid water. The Monte Carlo BEAMnrc/DOXYZnrc package was used to model beam generation and detection. The beam/detector model was validated with comparison to measured open field profiles. Low-Z target spectra differ significantly from the standard 6MV spectrum with Be-target and Al-target peaks at ∼ 25keV and ∼50keV, respectively. Approximately 1/3rd of the photon population is below 60keV for the 4MeV/Be beam. Planar contrast is increased significantly over that of 6MV using low-Z targets, showing additional improvement with removal of the detector's copper build-up layer. Contrast decreases with increasing separation more rapidly for 6MeV/Al than for 6MV; however contrast for the former is superior over the full range of separation examined. Use of low-Z linac target materials improves MV image contrast. An additional advantage is realized by removing the copper layer from the a-Si detector. This research has been sponsored by Varian Medical Systems, Incorporated.
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