Video-based system that gates the imaging and treatment beams on and off at predetermined intervals based upon the patient’s breathing pattern, and

also detects unexpected motion in any direction. The system uses an infrared tracking camera to track and analyze reflections from a marker block, and a patented predictive filter that monitors and predicts the patient's breathing pattern. The system analyzes motion based on the movement of external makers placed on the patient's chest. Dose delivery is permitted only during certain phases of the periodic movement, to minimize error caused from that motion. See also respiratory gating system; infrared camera; marker block; predictive block. Gating the treatment beam based upon respiration, as monitored with an infrared camera system.