A radiation therapy apparatus including a robot supporting a robot head a therapeutic radiation source attached to the robot head a collimator for adjusting a radiation field shape of therapeutic radiation radiated from the therapeutic radiation source a first therapeutic radiation detector attached to the robot head a couch configured to support a patient lying supine thereon a second therapeutic radiation detector for detecting the therapeutic radiation, disposed opposite the first therapeutic radiation detector with the couch disposed therebetween at least two X-ray sources and detectors for position detection of a marker and/or a treatment target an image processor for reconstructing an image of the treatment target and a CPU that computes the intensity, irradiation direction, dose, and dose distribution of the therapeutic radiation, and dose absorbed by the treatment target, radiation field shape, and position of the treatment target in real time for feedback to a next irradiation.
