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