Abstract: A radiation therapy apparatus, for a radiation therapy machine providing an irradiation of a tumor at 360.degree. about the tumor within a plane, determines a distribution of charges in a conductor that would produce a potential energy field matching the desired dose to the tumor in the plane. The fluence of any given ray through the tumor may be determined by summing the charges along the ray's path. The distribution may include areas of no irradiation which may require negative fluences. Physically realizable non-negative fluences are obtained by an iterative process of adjusting an input dose map in light of the actual dose produced by the calculated fluences.