1. A method, comprising: detecting, with a treatment delivery system, a treatment target having motion in up to three translational directions using direct registration of the treatment target, wherein detecting the treatment target comprises: generating one or more digitally reconstructed radiographs (DRRs); generating a 2D contour of the treatment target on each of the one or more DRRs; generating a region of interest (ROI) in each of the one or more DRRs, wherein the ROI includes at least a portion of the 2D contour of the treatment target; registering the ROI of each DRR with a corresponding in-treatment image; selecting a first in-treatment image that best matches a first ROI in a first corresponding DRR; generating a constrained search window in a second in-treatment image; and searching within the constrained search window to match the second in-treatment image and a second ROI of a second corresponding DRR; tracking the treatment target to synchronize a treatment beam with the motion of the treatment target.