A respiratory gating system is disclosed that varies the orientation of a radiation emitting device according to a patient's natural breathing. A breathing respirator is provided that allows the patient's respiration amount to be measured. External markers are adhered to triangulation points around a radiation target region, such as a heart, of the patient. An image diagnosis device images the target region. A computed tomography device reveals movements of the target region caused by the respiration. Triangulations and polynomial approximations are used to estimate the trajectory of the target region in real time. Position coordinates derived from the estimated trajectory are transmitted to the radiation emitting device. The system increases the accuracy and stability of the entire radiation therapy result.