Two-film brachytherapy reconstruction algorithm

Abstract

A new isocentric 2-film reconstruction algorithm was developed for brachytherapy [human cancer therapy] seed and needle implants. The algorithm has no requirements that the 2 films be orthogonal, symmetric, or even be taken in a transverse plane. In addition, there is no requirement that the 2 films even have the same number of images. Removal of these usual constraints were useful for head and neck implants where images are often obscured by patient anatomy. The inherent image matching ambiguities associated with traditional 2-film techniques are minimized by considering the image end points, rather than just the image centroids. For 2 films, the new algorithm, which considers all image combinations at 1 time, matches al the end-point images on 1 film with those on the other, and then reconstructs the end-point positions of the seeds. The algorithm minimizes the difference between the actual images and the projected images from the reconstructed seeds. The new 2-film image matching problem is shown to be equivalent to the well-known assignment problem. For an implant of N seeds, this equivalence allows the 2-film problem to be solved by an algorithm (ACM algorithm 548) that scales with a polynomial power of N, rather than N! as is usually assumed. An implant of N seeds can be matched and reconstructed in approximately (N/20)2 s on a VAX 11/780.