Abstract: An apparatus controls a stereotactic radiosurgery dose applied to an object being treated by a linear accelerator mounted on a gantry. The linear accelerator is capable of generating a beam having a path and a beam centerline, directed toward the object, and the beam is capable of having a plurality of angular positions relative to the object. The apparatus includes a turntable that is rotatable about an axis of rotation and defines a plurality of openings passing therethrough. Each opening has a center and receives therein a beam collimator of a preselected size. The turntable is disposed so that each of the plurality of openings may be rotated to where the center of the opening is coincident with the beam centerline, thereby allowing the beam to pass therethrough. A drive rotates the turntable about the axis of rotation and a sensor determines the angular position of the beam relative to the object.