1. A method of dynamically tracking a target within an anatomical region in order to deliver therapeutic radiation to the target during motion of the anatomical region, the anatomical region comprising one or more internal anatomical reference structures, the method comprising: constructing a 4D mathematical model that correlates relative 3D locations of the target and the one or more internal anatomical reference structures each identified in one or more pre-operative images, as a function of time within a periodic cycle P, wherein the periodic cycle P characterizes periodic motion of the anatomical region; after constructing the 4D mathematical model, dynamically tracking the periodic motion of the anatomical region to determine in real time a current point in time within the periodic cycle P; registering one or more near real time images of the one or more internal anatomical reference structures with one or more digitally reconstructed radiographs (DRRs) generated from the one or more pre-operative images of the one or more internal anatomical reference structures to determine a 3D location of the one or more internal anatomical reference structures in near real time within the periodic cycle P; and computing a 3D location of the target in real time using the 3D location of the one or more internal anatomical reference structures, and the relative 3D locations of the target and the one or more internal anatomical reference structures from the 4D mathematical model at the current point, wherein computing the 3D location of the target comprises determining vectors from the one or more internal anatomical reference structures to the target to identify the 3D location of the target.