Three-dimensional verification of patient placement during radiotherapy using portal images

The use of a single portal image for the three-dimensional verification of patient placement during radiotherapy has been investigated. In this study, a deviation in the patient positioning is quantified by a three-dimensional translation and rotation of the patient. The parameter values are obtained by fitting the projections of anatomical match points in the simulator and the portal image, using the three-dimensional gantry coordinates of these points in the intended treatment setup. Two methods for finding an analytical solution for the fitting problem are presented. One method yields a solution from the affine transformation of the portal image (shift, rotation, magnification, and stretching in a specified direction) that fits the projections of the match points on the simulator image. The second and more accurate method yields a solution that expresses the estimated parameter values and their covariances as functions of image and three-dimensional gantry coordinates of the selected matchpoints. The robustness and sensitivity of the solution is implicitly given by these expressions. The applications of these methods are illustrated by experiments with a human pelvic bone.