System that provides both respiratory gating for respiration-synchronized imaging and treatment, as well as 3D real-time patient position monitoring. Respiratory gating technology correlates the tumor position with the patient’s respiratory cycle. The system verifies the planned gated treatment with fluoroscopic imaging or a gated kV radiograph, by comparing those images against those taken by an infrared tracking camera of the movement of a marker block on a patient's chest. Comparison of images allows verifying that the target position is within the treatment field aperture for given gating thresholds. See also Real-time Position Management.