Towards Automatic, Model-Driven Determination of 3D Patient Setup Errors in Conformal Radiotherapy

The accurate and quantitative determination of three-dimensional patient setup errors in conformal radiotherapy, including setup errors due to out-of-plane rotations, requires methods for registering pre-treatment, three-dimensional planning CT images with intra-treatment, two-dimensional portal images. We have developed a method for performing such a registration based on structural models that emphasize medial aspects of shape. Such models (1) provide an ability to pre-select those structures in a reference image which are known to be reliable fiducials for registration, (2) allow for the stable recognition of the same structures in treatment portal images, and (3) can be combined with images in such a way as to yield a measure of agreement between the model and the features in the image. We describe the means for creating a model in a reference image generated from the planning CT, for deforming the model to identify corresponding structures in a treatment portal image, and for optimizing an objective function based on combining the deformed model with a collection of digitally reconstructed radiographs generated from CT at tentative poses. The optimum of the objective function yields the three-dimensional pose of the patient relative to the planning pose, thereby indicating the three-dimensional setup error. Pilot results using simulated images with known patient positioning errors have shown that such an objective function obtains an optimum very near to truth. Correspondence to: Paul Yushkevich Sitterson Hall, CB# 3175 The University of North Carolina at Chapel Hill Chapel Hill, NC 27599-3175 email: [email protected] MICCAI ’98 Review Draft Yushkevich et al.