Abstract: In a high energy X-ray therapy machine, an accelerator accelerates a beam of electrons to energies of several MEV. The electron beam is directed, generally via a beam deflector, onto an X-ray target to produce X-rays for treating a patient. The accelerator and beam deflector are often carried within a rotatable gantry structure, movable for radiating the patient through a series of angular positions or ports. A plurality of ionization chamber radiation detectors are disposed in the radiation field surrounding the beam path of the accelerated electron beam. Ionization current signals are compared from a plurality of the detectors to derive an error signal determinative of a deviation of the beam from an intended beam path. The error signal is utilized to energize beam steering coils for steering the beam along the predetermined beam path.