1. A method for positioning a patient for radiotherapy, comprising: acquiring a first x-ray image sequence of a target inside the patient by projecting a radiation beam from a linear accelerator positioned 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 by projecting a radiation beam from the linear accelerator positioned at a second angle; acquiring second respiratory signals of the patient while acquiring the second x-ray image sequence, wherein the second x-ray image sequence and second respiratory signals are acquired after the first x-ray image sequence and first respiratory signals; retrospectively synchronizing the first and second respiratory signals with each other 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.