Chapter 100 CyberKnife Radiosurgery for Spinal Neoplasms
The successes of cranial stereotactic radiosurgery (SRS) inspired the development of spinal SRS.1–4 In little more than a decade, spinal SRS has revolutionized the treatment of spinal tumors and vascular malformations. The introduction of CyberKnife™ has allowed the delivery of very large doses of radiation to small lesions while sparing adjacent normal structures. CyberKnife™ outcomes are comparable or superior to those obtained with conventional radiotherapy, frame-based stereotactic systems, or conventional surgery.1–13 Since 1994, the CyberKnife™ has been used to treat over 10,000 spinal lesions at more than 200 sites worldwide.
Technology Overview
The CyberKnife system consists of a lightweight, 6-megavolt (MV) linear accelerator (LINAC) mounted on an industrial robot, a remotely repositionable treatment couch, orthogonally placed digital x-ray cameras, a treatment-delivery computer, and treatment planning stations (Fig. 100-1). During treatment, numerous images are obtained to optimally locate the target before the delivery of 100 to 150 individual treatment beams.14 The CyberKnife can deliver individual beams to any part of a tumor from nearly any angle and provides highly conformal dosing to complex three-dimensional (3D) targets.15

FIGURE 100-1 A, The CyberKnife frameless stereotactic system includes a modified 6-MV X-band LINAC mounted on a highly maneuverable robotic manipulator (KUKA Roboter GmbH, Augsburg, Germany). B, Two high-resolution x-ray cameras are mounted orthogonally to the headrest. C, One of the two x-ray sources is mounted in the ceiling projecting onto the camera. D, The treatment couch is mobile, allowing the x-ray sources to image targets at any point along the neuraxis.
Treatment Details
Spinal SRS is image guided and completely frameless. Individually molded masks or cradles are fashioned before the planning scans are completed. The patient rests in the mask or cradle during computed tomography (CT) scanning, magnetic resonance imaging (MRI), 3D angiography or positron emission tomography (PET) imaging, and again during treatment. The devices are comfortable, limit movement, and expedite positioning. At Stanford, we use an Aquaplast mask (WFR Corp., Wyckoff, NJ) for upper cervical lesions (Fig. 100-2A). Lower cervical, thoracic, lumbar, and sacral lesions are treated using an AlphaCradle (Smithers Medical Products, Akron, OH) (Fig. 100-2B). Most patients are treated supine but prone and lateral decubitus positioning is possible.

FIGURE 100-2 Simple immobilization devices used during CyberKnife treatment. A, The Aquaplast mask is used in patients with upper cervical lesions. B, AlphaCradle custom body mold is used in patients with lesions below the cervical spine.
Bony landmarks are used to target spinal lesions and those in adjacent structures. Spinal fusion hardware does not interfere with treatment. The accuracy of CyberKnife approaches ±0.5 mm.7,16 For lesions not associated with bony landmarks, three or more gold seeds or titanium screws are implanted (Fig. 100-3).13,15

FIGURE 100-3 Implanted fiducials are marked and numbered. Left, Computed tomography-based digitally reconstructed images from the perspective of the two orthogonal CyberKnife mounted x-ray cameras (A and B). Center, Real-time x-ray images from the two x-ray cameras. Right, Overlay of the reconstructed and actual radiographic images.
CyberKnife treatment plans use Accuray’s MultiPlan software. BrainLab, Varian, and Elekta systems employ similar programs. After uploading the CT and other images to the planning station, the surgeon and radiotherapist “contour” the target and radiation-sensitive structures by tracing their outlines (Fig. 100-4). The dose and number of sessions, as well as dose limits for adjacent structures, are prescribed by the physicians (Fig. 100-5). Physicists then use the planning system, the contour data, and the dose prescriptions to create a 3D representation of the lesion geometry and define sets of treatment beams. An ideal treatment includes evenly distributed beams that target the dose uniformly while limiting exposure to sensitive structures (Fig. 100-6). The neurosurgeon and/or physicist will iteratively perfect the plan by adding or removing constraints, or re-positioning individual beams. A multidisciplinary team reviews and accepts each treatment plan before delivery.

FIGURE 100-4 Contour of L3 metastasis in axial, saggital, and coronal projections. The epidural metastasis is in red.

FIGURE 100-5 Contour of L3 metastasis and spinal roots with superimposed isodose lines from treatment plan in axial, saggital, and coronal projections. The epidural metastasis is in red, the spinal roots are blue, and the 80% isodose line is represented by the thin green line.
Indications
It cannot be emphasized enough that the indications for spinal SRS continue to evolve quickly; spinal SRS is a new subdiscipline within neurosurgery and only now are standardized procedures for its application being developed. At our institution, we most frequently treat metastases, small benign tumors, postoperative residuals, lesions that recur following conventional surgery or radiation, vascular malformations, inoperable tumors, and lesions in those who decline surgery (Tables 100-1 and 100-2).3,7,13 Spinal lesions appropriate for CyberKnife™ radiosurgery should be reasonably well circumscribed, clearly visible on CT or MRI, and smaller than approximately 5 cm in diameter. We do not insist on obtaining a biopsy in advance of treatment if the diagnosis is clear from preradiosurgical imaging studies.
Table 100-1 Indications and Contraindications for Stereotactic Spinal Radiosurgery
Indications | Contraindications |
---|---|
Progressive but minimal neurologic deficitPostresection or post-RT local irradiation (boost)Disease progression after surgery and/or irradiationInoperable lesions or high-risk lesion locationsMedical comorbidities that preclude surgeryLesions in patients who decline surgery | Spinal instability (adjunctive treatment only)Neurologic deficit caused by bony compressionSevere neurologic deficit due to cord compressionAdjacent cord previously radiated to maximum doseVery rare lesions not responsive to ionizing radiation |
Table 100-2 Treated/Treatable Lesions with CyberKnife Radiosurgery
Tumors |
Benign |
Neurofibroma, schwannoma, meningioma, hemangioblastoma, chordoma, paraganglioma, ependymoma, epidermoid |
Malignant/Metastatic |
Breast, renal, non–small-cell lung, colon, gastric and prostate metastases; squamous cell (laryngeal, esophageal, and lung) tumors; osteosarcoma; carcinoid; multiple myeloma; clear cell carcinoma; adenoid cystic carcinoma; malignant nerve sheath tumor; endometrial carcinoma; malignant neuroendocrine tumor |
Vascular Malformations |
Arteriovenous malformation (types 2 and 3) |
Extradural Metastases
The spine is the most common site for bony metastases, accounting for nearly 40% of osseous tumor spread.17 Forty percent of cancer patients will develop at least 1 spinal metastasis.18 Historically, spinal metastases have been managed with chemotherapy, radiopharmaceuticals, surgery, and external beam irradiation.17,19 Conventional irradiation of spinal metastatic tumors is useful for palliation but its effectiveness is limited by spinal cord tolerance.20 Moreover, relapses are common,21–24 and retreatment with RT is generally impossible.18 SRS enables much larger biologically effective doses to be delivered by utilizing a more highly conformal plan that protects the cord. Multiple courses of spinal SRS can control multiple asynchronous metastases, and SRS may be used to sterilize a vertebral body before vertebroplasty25 or following a debulking procedure. The presence of spinal fusion hardware is not a contraindication.26