Megavoltage image contrast with low-atomic number target materials and amorphous silicon electronic portal imagers

Low-atomic number (Z) targets have been shown to improve contrast in megavoltage (MV) images when using film-screen detection systems. This research aims to quantify the effect of low-Z targets on MV image contrast using an amorphous silicon electronic portal image detector (a-Si EPID) through both experimental measurement and Monte Carlo (MC) simulation. Experimental beams were produced with the linac running in the 6 MeV electron mode and with a 1.0 cm aluminum (Al, Z = 13) target replacing flattening filtration in the carousel, (6 MeV/Al). A 2100EX Varian linac equipped with an aS500 EPID was used with the QC3 MV phantom for the majority of contrast measurements. The BEAMnrc/EGSnrc MC package was used to build a model of the full imaging system including beam generation (linac head), the a-Si detector and the contrast phantom. The model accurately reproduces contrast measurements to within 2.5% for both the standard 6 MV therapy beam and the 6 MeV/Al beam. The contrast advantage of 6 MeV/Al over 6 MV, as quantified with the QC3 phantom, ranged from a factor increase of 1.6 +/- 0.1 to 2.8 +/- 0.2. Only a modest improvement in contrast was seen when the incident electron energy was reduced to 4 MeV (up to factor of 1.2 +/- 0.1 over 6 MeV/Al) or with removal of the copper build-up layer in the detector, (up to factor of 1.2 +/- 0.1 over 6 MeV/Al). Further decreasing the target Z, to beryllium (Be, Z = 4), at 4 MeV showed no significant improvement over 4 MeV/Al. Experimentally, the contrast advantage of 6 MeV/Al over 6 MV was found to decrease with increasing patient thickness, as can be expected due to selective attenuation of low-energy photons. At head and neck-like thicknesses, the low-Z advantage is a factor increase of 1.7 +/- 0.1.