SIMILARITIES AND DIFFERENCES IN RADIATION-THERAPY OPTIMIZATION AND TOMOGRAPHIC RECONSTRUCTION

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