1. A method to locate and track a moving, three-dimensional (3D) target in a patient undergoing image-guided radiation therapy, the method comprising: (a) receiving an adaptive filter model for a region of interest in the patient, wherein the adaptive filter model is based on the target to be tracked; (b) obtaining, by an image acquisition device, a two-dimensional (2D) slice of a region of interest in the patient; (c) applying, by a processor, the adaptive filter model to the 2D slice, wherein the adaptive filter model includes an offset value; (d) determining, by the processor, a location of the target in the 2D slice based on the adaptive filter model; (e) estimating, by the processor, a potential location of the target based on the offset value; and repeating, by the processor, (b) through (e) to track the moving target during image-guided radiation therapy of the patient.