We describe here a new method of calculating the target point magnification rate for radiosurgery by stereotactic angiography using a Leksell frame. As the distance (D) between the x-ray focus and the film plane is constant in each facility, the magnification rate (M) is determined by the equation: M = D⁄[D⁄Mp + t], where Mp is the magnification rate of the proximal (to the x-ray focus) frame pro jected on the film plane and t is the distance between the proximal frame plane and the target

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... . The t value is given by the three-dimensionally determined stereotactic coordinates of the target point. Only the Mp value on the stereotactic angiogram must be measured. This method is theoretically more correct and has less risk of human error than the conventional, graphically determined method which necessitates measuring both the distal and proximal frame magnification rates. Furthermore, the in herent theoretical error of the conventional method is largest around the frame center, which is fre quently selected as the target point for radiosurgical treatment, possibly causing serious error in ac curate dose planning.