A target motion simulator system for use in verifying target tracking with a radiation therapy device. The system comprises a radiation detection target coupled to a first motion actuator simulating a first motion of a first tissue and a fiducial coupled to a second motion actuator simulating a second motion of a second tissue offset from the first tissue, a component of the first motion being asynchronous with the second motion. A synthetic physiological signal generator is synchronized with the component of the first motion, wherein an output signal from the generator, in combination with a sensed position of the fiducial, may be used by the radiation therapy device in tracking the target.
