A set of 2D reconstructed images is generated 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. Said set of 2D reconstructed images are registered 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. A positioning system is instructed 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.
