ASSESSMENT OF THE ACCURACY OF STEREOTAXIC RADIOSURGERY USING FRICKE-INFUSED GELS AND MRI

The treatment plans for stereotactic radiosurgery employ small, circul ar, noncoplanar fields applied in a series of arcs, or with synchronou s rotation of the accelerator gantry and patient support assembly. Pri mary 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 treatm ent 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 targ et that was localized by both CT and MRI. A stereotactic head frame wa s 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 loca lized by CT and MRI. The head frame was then mounted to the patient su pport assembly of a linear accelerator, and given a dose of 40 Gy to t he isocenter from 6-MV x rays using a modified version of the dynamic stereotactic radiosurgery plan developed in Montreal. Subsequent MRI s howed the target point at the center of the dose distribution, thus co nfirming 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.