1. A medical imaging-based method for organ motion tracking as a function of time, the method comprising: a) capturing kV and MV images during a treatment period using a kV imaging detector and a MV radiotherapy beam imaging detector; b) extracting 2D locations of internal features from the captured kV and MV images; and c) computationally tracking 3D geometric locations of the internal features as a function of time from the extracted 2D locations of the internal features and information describing geometry of the kV imaging detector and MV radiotherapy beam imaging detector.