1. A method for representing a treatment site on a patient, the treatment site containing a surface, as data suitable for use with a computer-aided design device, the method comprising the steps of: scanning the treatment site with a scanning apparatus to produce image data for a plurality of voxels; identifying a plurality of boundary voxels representative of the surface of the treatment site; generating a vector representing the location of each boundary voxel; and converting the vectors into a form of data suitable for use with a computer-aided design device by associating each of the vectors with a pole of a control polygon of a non-uniform rational B-spline.