Sci-Fri AM General-04: High Contrast Imaging Using Orthogonal Bremsstrahlung Beams: An Experimental Study of Radiation Quality

Abstract

Since portal images are created by megavoltage, forward‐directed bremsstrahlung beams, their image quality is inferior to that of images produced by kilovoltage beams. In this study, characteristics of orthogonal bremsstrahlung photons produced by megavoltage electron pencil beams were studied and suitability of their use for improved radiotherapy imaging was evaluated. A 10 MeV electron beam emerging through the research port of a Varian Clinac‐18 linac was made to strike targets of carbon,aluminum and copper. PDD and attenuation measurements of both the forward and orthogonal beams were performed, and experimental results were compared with Monte Carlo‐calculated findings. Images of contrast objects were acquired with Agfa 400 diagnostic films and their contrast levels were analyzed. Photon yield and effective energy are lower for orthogonal beams than for forward beams, and the differences are more pronounced for targets of lower atomic number. The effective energy of a spectrum produced by carbon dropped by a factor of 10 from 1535 keV in the forward direction to 151 keV in the orthogonal direction, while for aluminum the effective energy dropped by 77% to 425 keV, and for copper by 37% to 1107 keV. The imagecontrast of films exposed with orthogonal beams was qualitatively determined to be superior to that obtained using the forward megavoltage beams. Using their relatively low effective energy, orthogonal bremsstrahlung beams produced by megavoltage electrons striking low atomic number targets yield images with a higher contrast than do forward bremsstrahlung beams.