The problem of body inhomogeneities in radiation therapy

Since the introduction of accurate treatment planning techniques into radiation therapy, the presence of regions of non unit density tissues within the patient has interfered with accurate estimates of radiation doses delivered to the tumor and surrounding normal tissues. Various methods of predicting the location and extent of these inhomogeneous regions, and of estimating the influence of the regions on dose distributions computed for a unit density homogeneous medium, have largely been unsuccessful. These difficulties now appear to be resolvable, at least in institutions with capability for whole body computed tomography, a capability that one can attain by purchasing a commercial diagnostic x ray computed tomographic unit, with correction of computed attenuation coefficients to values appropriate for the type and energy of radiation to be employed for therapy. Alternately, a computed tomographic unit that employs high energy photons and is dedicated solely to treatment planning appears feasible. With such a unit, no correction of computed attenuation coefficients is necessary before entry of the data into the treatment planning computer.