Technical description, phantom accuracy, and clinical feasibility for fiducial-free frameless real-time image-guided spinal radiosurgery

Object. The authors describe the technical application of the Xsight Spine Tracking System, data pertaining to accuracy obtained during phantom testing, and the initial clinical feasibility of using this fiducial-free alignment system with the CyberKnife in spinal radiosurgery. Methods. The Xsight integrates with the CyberKnife radiosurgery system to eliminate the need for implantation of radiographic markers or fiducials prior to spinal radiosurgery. It locates and tracks spinal lesions relative to spinal osseous landmarks. The authors performed 10 end-to-end tests of accuracy using an anthropomorphic head and cervical spine phantom. Xsight was also used in the treatment of 50 spinal lesions in 42 patients. Dose planning was based on 1.5-mm-thick computed tomography slices in which an inverse treatment planning technique was used. All lesions could be treated using the fiducial-free tracking procedure. Phantom tests produced an overall mean targeting error of 0.52 +/- 0.22 mm. The setup time for patient alignment averaged 6 minutes (range 2-45 minutes). The treatment doses varied from 12 to 25 Gy to the median prescription isodose of 65% (40 to 70%). The tumor volume ranged between 1.3 and 152.8 cm(3). The mean spinal cord volume receiving greater than 8 Gy was 0.69 +/- 0.35 cm(3). No short-term adverse events were noted during the 1- to 7-month follow-up period. Axial and radicular pain was relieved in 14 of 15 patients treated for pain. Conclusions. Fiducial-free tracking is a feasible, accurate, and reliable tool for radiosurgery of the entire spine. By eliminating the need for fiducial implantation, the Xsight system offers patients noninvasive radiosurgical intervention for intra- and paraspinal tumors.