The treatment plans for stereotactic radiosurgery employ small, circular, noncoplanar fields applied in a series of arcs, or with synchronous rotation of the accelerator gantry and patient support assembly. Primary or metastatic brain tumors and arterial-venous malformations are localized in relation to a stereotactic head frame using CT, MRI, and angiography. As x-ray doses in the range of 20-40 Gy are delivered in a single treatment, it is critical that the dose distribution produced by the accelerator accurately reflect the one developed by the treatment planning computer. Until the advent of Fricke-infused gels, whose NMR characteristics are changed by irradiation, there was no practical method for assessing the accuracy of x-ray beam positioning on a target that was localized by both CT and MRI. A stereotactic head frame was attached to a hollow glass head filled with a Fricke-infused gel. A 2-mm target point at approximately the center of this manikin was localized by CT and MRI. The head frame was then mounted to the patient support assembly of a linear accelerator, and given a dose of 40 Gy to the isocenter from 6-MV x rays using a modified version of the dynamic stereotactic radiosurgery plan developed in Montreal. Subsequent MRI showed the target point at the center of the dose distribution, thus confirming the accuracy of the stereotactic radiosurgery procedure. This demonstrated the unique characteristics of the Fricke-infused gel for the simultaneous localization of x-ray beams in three dimensions.