SIMILARITIES AND DIFFERENCES IN RADIATION-THERAPY OPTIMIZATION AND TOMOGRAPHIC RECONSTRUCTION

Modern photon radiotherapy optimization methods require the use of a number of nonuniform dose distributions incident on the tumor. From this point of view, radiotherapy optimization has strong similarities with the reconstruction problem in tomographic imaging. In general, the image reconstruction problem is simpler because in the absence of noise and with sufficiently many projections an exact solution always exists. However, it is in general impossible by external beam irradiation to produce an arbitrary desired dose distribution in the patient. This is primarily because the order of events from physical collection of projection data to reconstruction theory is reversed in therapy optimization, starting with the theory and ending with physical irradiation, where negative dose delivery is impossible. Despite this fundamental problem, many approximate image reconstruction methods work quite well for therapy optimization even though strict optimization requires radiobiological models and the finest external beam radiation tool available: the pencil beam. (C) 1995 John Wiley and Sons, Inc.