Sci-Thurs PM: Delivery-09: Improving megavoltage portal image contrast with low atomic number target materials

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 1/3^rd 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.