A P-THETA TECHNIQUE FOR TREATMENT VERIFICATION IN RADIOTHERAPY AND ITS CLINICAL-APPLICATIONS

A new technique based on a rho-theta coordinate system for determining differences position between portal and simulator images is presented . Unlike the conventional point matching method, which requires the fi ducial points to be labeled in pairs before the registration, the rho- theta technique avoids this manual procedure. It accomplishes the trea tment verification in two major steps; image alignment and field displ acement analysis. For the same number of fiducial points in the simula tor and portal images, it first finds the corresponding paired points if the points are not distributed symmetrically about their centroid. This is followed by alignment of these paired points using the least s quares matching method to find the optimal two-dimensional rigid body transformation parameters (shift, rotation, and scaling factor). The t ransformation parameters are then used to transform the portal field e dge into the simulator image, so that the portal field can be compared with the prescribed field on the simulator image. A number of paramet ers were explored to describe the field displacement errors, including treatment field size, under/over irradiated size, the shift in center of gravity of the field, the field edge shift, and rotation of the fi eld. The rho-theta technique as implemented is both fast and accurate. Experiments on the registration of radiological phantom portal images acquired with an on-line portal imaging system mounted on a linear ac celerator indicate an accuracy on the order of 1 mm in detecting the s hift of the field's center of gravity and approximately 1-degrees in d etecting the field rotation. The results of a clinical trial are also presented. The technique appears well suited as an integral part of an automated on-line portal imaging treatment verification system.