Introduction

Stereotactic radiosurgery (SRS) for the spine is a noninvasive technique that accurately delivers large doses of radiation to small targets, through the use of numerous highly collimated cross-fired beams. Spinal SRS has proved effective in treating most metastases, many benign intradural tumors, some intramedullary tumors, and compact intramedullary arteriovenous malformations. It can be used for the older patient or in patients with widely metastatic disease, when open surgery might be contraindicated.

Radiosurgery was developed in 1949 by Lars Leksell and Bjorn Larsson at the Karolinska Institute in Stockholm. Their first system used orthovoltage x-rays, and a subsequent variant used a proton beam generated by a cyclotron. Leksell’s Gamma Knife was introduced in 1967, as a lower cost and more efficient system. Gamma Knife treatments provided a steeper dose gradient outside the target region. Embedded in a cast-iron enclosure, the device contained 201 radioactive cobalt sources focused at a single point or isocenter. During treatment, the patient’s head was secured to a rigid frame with pins and then inserted into the cast-iron device with the lesion positioned at the isocenter. In the mid-1980s, to make radiosurgery more accessible and less costly, Betti and Colombo modified conventional radiotherapy linear accelerator-based (LINAC) systems to deliver frame-based radiosurgery. Although both the Gamma Knife and LINAC systems were powerful tools for intracranial disease, they could not be easily adapted for extracranial cases, primarily because of the necessity for frame-based target localization.

In 1991, the frameless CyberKnife system was developed by Adler at Stanford University. From its inception, this device was intended to treat both cranial and extracranial lesions with sub-millimeter accuracy. Since the installation of the first clinical unit in 1994, over 8000 spinal lesions have been treated at more than 180 worldwide CyberKnife sites. As in cranial radiosurgery, spinal SRS delivers large but precise doses of radiation to a target while sparing adjacent healthy tissues. In comparison, conventional radiotherapy doses are limited by the sensitivity of the spinal cord.

Radiosurgery

Ionizing radiation damages DNA, protein, and lipids by creating free radicals and causing either mitotic or apoptotic cell death. Larger doses, while more effective in killing neoplastic tissues, endanger normal structures. Conventional radiotherapy, which uses small numbers of broad and relatively inaccurate beams, addresses the problem by dividing the dose over many daily treatments. In contrast, SRS instruments can deliver scores of small, precisely collimated beams. As a consequence, large radiation doses can be directed at irregularly shaped lesions while avoiding adjacent radiosensitive tissue. The target’s location and shape are delineated using computed tomography (CT) or magnetic resonance (MR) images, and processed using dedicated treatment-planning software. Current radiosurgery systems capable of treating spinal lesions include the CyberKnife, the Tomoscan (a CT-like device) and various modified linear accelerators (LINACs). The Gamma Knife is currently able to treat only select upper cervical lesions.

The CyberKnife system is a completely frameless, image-guided, robotic radiosurgery system which consists, in part, of a lightweight, six megavolt

Clinical Case Examples

Case 1

MC, a 66-year-old woman with medical contraindications to open surgery, was treated with SRS for a spinal schwannoma. She initially presented with a chronic cough and generalized weakness. Routine laboratory studies showed pancytopenia, and flow cytometry was consistent with acute lymphocytic leukemia. The diagnosis was confirmed by bone marrow biopsy. While undergoing chemotherapy in December 2008, she developed lower back pain with subjective right leg weakness. A 10-by 7-mm epidural lesion, compressing the S1 nerve root, was seen on MRI (Figure 49-1). A CT-guided biopsy was consistent with schwannoma, but definitive treatment was postponed because of her leukemia. By July 2009, the pain became intolerable. She had 4/5 gastrocnemius weakness, numbness in the S1 distribution, and loss of the ankle reflex. A repeat MRI confirmed an increase in the size of the schwannoma. Open surgery remained high risk, so in September 2009, the patient underwent CyberKnife treatment. She received 16 Gray (Gy) in a single session. Pain complaints improved, and all examination findings resolved over 2 months.

FIGURE 49-1 Gadolinium-enhanced T1-weighted axial image demonstrating a 10- by 7-mm intradural extramedullary lesion compressing the S1 nerve root. A CT-guided biopsy was consistent with schwannoma.

Case 2

WF is a 68-year-old man with melanoma who received SRS treatment for a recurrent spinal metastasis in a previously irradiated field. The patient was initially seen with a melanoma of the back in 1999 and another of the neck in 2003. Following local resections, he remained disease-free until March 2007, when, after complaining of low back pain, he was found to have a 4 × 3 cm lesion of the L3 vertebral body. There was significant compression of the cauda equina due to epidural extension. A PET-CT showed hypermetabolic areas in the lung, bone, and brain. He received conventional radiotherapy of 37.5 Gy to the brain and 37.5 Gy to the lumbar spine. In September 2009, the patient returned with increasing back pain, associated with weakness. His strength was 4/5 in the right leg but his sensation was intact. A follow-up PET-CT and MRI showed multiple new lesions and enlargement of the previously treated L3 mass (Figure 49-2). Additional conventional radiotherapy was not an option and a surgical decompression was contraindicated based on his other medical problems. The L3 lesion was treated with CyberKnife SRS, 24 Gy in three sessions. His symptoms improved.

FIGURE 49-2 Sagittal T1-weighted image with contrast, demonstrating enlargement of the previously treated L3 vertebral body metastasis with epidural extension causing central canal stenosis.

linear accelerator attached to an industrial robot (Figure 49-3). The robotic arm is unconstrained, using six degrees of freedom to deliver beams to virtually any part of the body from a wide range of angles. During treatment, real-time orthogonal images of the patient are obtained frequently, enabling the system to identify and automatically correct for small changes in patient position.

FIGURE 49-3 CyberKnife frameless stereotactic radiosurgery suite. A modified 6-MV X-band LINAC designed specifically for radiosurgery is mounted on a highlymaneuverable robotic manipulator (KUKA Roboter GmbH, Augsburg, Germany) (A). Two high-resolution x-ray cameras are mounted orthogonally to the headrest (B). One of the two x-ray sources is mounted in the ceiling projecting onto the camera (C). The treatment couch is mobile, allowing the x-ray sources to image targets at any point along the neuraxis (D).

Several conventional radiation therapy systems have been modified to provide spinal SRS. The BrainLab Novalis and TX systems both use floor- and ceiling-mounted x-ray cameras to verify patient position during therapy. In contrast, the Varian Trilogy and Elektra Synergy systems utilize cone-beam CT scanners mounted on the gantry of the LINAC. The cone CT scanners acquire images before treatment, but do not do so regularly during each session, and cannot always accommodate for changes in patient movement during therapy.

Indications for Spinal Radiosurgery

Indications for spinal SRS continue to evolve (Tables 49-1 and 49-2). The most commonly treated spinal lesions are metastatic (Table 49-3). A biopsy may not be necessary prior to treatment if the diagnosis is clear from the clinical history and imaging. Ideally, lesions should be less than 5 cm in maximal diameter, well demarcated, and clearly seen on CT and/or MRI. For most tumors, local control rates are equivalent or superior to conventional radiation and complications are generally lower than with open surgery. In some particular cases, spinal SRS may be useful for ablating the more radioresistant tumors.¹ However, in those previously irradiated patients where the adjacent spinal cord has already received the maximum tolerated radiation dosage, the efficacy of spinal radiosurgery may be compromised because of the need to lower the radiosurgical dose.

TABLE 49-1 Indications for Spinal SRS

Tumors that are highly radiosensitive.

Post-resection cavity

Post-radiation therapy local irradiation

Recurrent disease post surgery and/or irradiation

Inoperable lesion

High-risk location of lesion

Slowly progressive but minimal neurological deficits

Patient with medical comorbidities that preclude surgery

Patient declines surgery.

TABLE 49-2 Contraindications for Spinal SRS

Spinal instability