1. A radiation therapy system for delivering radiation to a mammal comprising: a radiation therapy source for delivering said radiation to a specified target in said mammal; a scanning beam x-ray source for providing x-rays from a plurality of x-ray focal spots directed to a field of view and said target within said field of view; a x-ray detector, with an area smaller than an area for said field of view, for measuring amount of said x-rays passing through said target and striking said detector; a tomosynthesis processor coupled to said detector for combining views corresponding to each of said focal spots to generate an image of said target; and a tracking controller coupled to said tomosynthesis processor for controlling said radiation from said radiation therapy source to said target.