A spherical acrylic phantom was designed for quality assurance measurements of dynamic radiosurgery. The phantom consists of two mating hemispheres mounted on a base plate. The interhemispheric plane may be oriented at any angle to the base, the angle being identified by visible marks on the base plate of the phantom. The phantom has a set of replaceable, radiologically identifiable markers, suitable for Computed Tomography (CT), Magnetic Resonance (MR), and Digital Subtraction Angiography (DSA) imaging. The frame coordinates of each marker are calculated from its known positions with respect to the center of the sphere. The measured errors of these positions using CT and MR images, were within the voxel size of the displayed image, while for DSA images the error was greater than 2.5 mm at the periphery of the image. The calculated depths from the planning software, for various beam intersection points to the isocenter, agreed within 0.6 mm with the known depths. A variation of 3.6 +/- 2.6 mm in the calculated depths was observed between using MR and CT image data. This difference results in a 1% variation in Tissue Maximum ratio (TMR) calculations. Comparisons of measured and known volumes resulted in differences of 8%-10%.