1. A method for positioning a patient for radiotherapy, comprising: acquiring a first x-ray image sequence of a target inside the patient at a first angle; acquiring first respiratory signals of the patient while acquiring the first x-ray image sequence; acquiring a second x-ray image sequence of the target at a second angle; acquiring second respiratory signals of the patient while acquiring the second x-ray image sequence; synchronizing the first and second x-ray image sequences with the first and second respiratory signals to form synchronized first and second x-ray image sequences; identifying the target in the synchronized first and second x-ray image sequences; and determining three-dimensional (3D) positions of the target through time in the synchronized first and second x-ray image sequences.