Improved contrast in radiation therapy imaging using low-Z targets and amorphous silicon portal imagers.

摘要

Prior to or during external beam radiation therapy, a megavoltage (MV) treatment beam may be used to image patient anatomy and to verify patient position. MV projection images, however, offer poor contrast, leading to possible uncertainty in patient positioning.;This study thus presents rationale for an improved method of beam-generation for MV imaging in radiation therapy using an a-Si EPID. The approach should be readily applicable to either planar or cone-beam imaging for image-guided radiation therapy (IGRT) techniques.;Low atomic number (Z) targets have been shown to alter the MV beam spectrum and to improve contrast in megavoltage images when using film-screen detection systems. Film-screen detection systems are no longer used in daily radiation therapy imaging, having been replaced by amorphous silicon electronic portal image detectors (a-Si EPID). 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 a clinical linear accelerator operated in the 6 MeV electron mode with a 1.0 cm-thick aluminum (Z = 13) target, (6MeV/Al) and contrast quantified using a standard MV imaging phantom. A MC model of the full imaging system was developed, including beam generation, a contrast phantom and the a-Si detector. The model is shown to accurately reproduce contrast measurements to within 2.5% for both the standard 6 MV therapy beam and the 6 MeV/Al beam. Using the 6 MeV/Al beam instead of the 6 MV beam, imaging contrast increases by a factor of 1.57 to 2.81. The MC model was then extended to predict the effect on contrast of decreasing incident electron energy to 4 MeV and of removing the copper build-up layer in the detector. Both of these changes produced further increases in image contrast. Further decreasing the target Z to beryllium (Be, Z = 4) showed slight improvement over Al at 4 MeV and 6 MeV. Experimentally, the contrast advantage of 6 MeV/Al over 6 MV was found to decrease with increasing separation, but at head and neck separations of approximately 15 cm of water, a factor increase in contrast of 1.7 remains.