1. A method comprising: generating a set of 2D reconstructed images from a pretreatment 3D scan showing an initial position of a target, wherein the set of 2D reconstructed images corresponds to perturbations from said initial position along fewer than six degrees of freedom; registering said set of 2D reconstructed images with one or more 2D x-ray images of said target showing a current position of the target, wherein the registering includes computing a set of 3D transformation parameters that represent a change in position of said target between said initial position of said pretreatment 3D scan and said current position of said 2D x-ray images along six degrees of freedom; and instructing a positioning system to adjust a relative position of a radiosurgical beam generator to said target by an amount prescribed by said 3D transformation parameters, wherein said target is allowed six degrees of freedom of position.