Stereotactic Radiosurgery of the Spine

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Chapter 16 Stereotactic Radiosurgery of the Spine

INTRODUCTION

Tumors and vascular malformations of the spine are often difficult to treat safely with conventional surgery. Notwithstanding their intimate relationship to delicate neuronal structures, access to these lesions can be limited and may be associated with significant operative risk. Other treatment strategies such as chemotherapy and irradiation have been used in conjunction with surgery to achieve local control. However, these strategies often do not provide substantial therapeutic benefit, and the inability to limit their effects to the target tissue can lead to frequent complications.

The standard radiation treatment field for spinal tumors generally includes one to two vertebral segments above and below the lesion to compensate for targeting errors and patient movement (Fig. 16-1). More importantly, the close proximity of the spinal cord to the lesion typically makes it impossible to exclude the spinal cord from the treatment field. In an effort to protect the spinal cord and other healthy tissue, conventional radiation therapy is highly fractionated to allow time for these tissues to recuperate from damage caused during treatment. However, despite such attempts, adequate therapeutic doses are often unobtainable because of the low tolerance threshold of the spinal cord to radiation.1 Consequently, relapse within the treatment field after standard radiotherapy is not uncommon.25

Shaping the irradiation beam to strictly conform to the target volume enables maximal dose delivery to the lesion while minimizing the risk of injury to the spinal cord. One attempt to achieve this goal was the development of intensity modulated radiation therapy (IMRT), which uses dynamic multi-leaf collimators to modify the intensity of the radiation as it is distributed to the tumor.6,7 This technology enables the shape of the treatment field to more closely resemble that of the lesion. However, most of the advantages of this technique go unrecognized because the method of radiation targeting remains relatively imprecise. During conventional radiotherapy, the radiation beams are aimed at the lesion by means of surface markers attached to the skin. The inherent nature of this process results in limited spatial accuracy and reproducibility. Consequently, even the most modern radiation therapy technique fails to address the unique demands of optimal radiation delivery to spinal tumors. Ultimately, the dilemma with all radiotherapy methods is that greater success can only be achieved at the cost of substantially increasing the risk of injury.

EVOLUTION OF SPINAL STEREOTACTIC RADIOSURGERY

Stereotactic radiosurgery uses the principles of stereotactic localization in combination with multiple radiation beams from a highly collimated source to deliver high-dose radiation to a target while limiting exposure to healthy tissues.8 This technology traditionally has been used to treat intracranial lesions and is based on the presumed fixed relationship between the target and skull that serves as a reference for target localization. During the past several years, the success of stereotactic radiosurgery has redefined the standards of treatment for intracranial pathology and has inspired the development of similar techniques for the treatment of spinal tumors. Lesions of the spine have essentially a fixed spatial relationship to the surrounding spinal column. Therefore, similar to early intracranial radiosurgery devices such as the Gamma Knife (Elekta, Atlanta, GA), the initial development of spinal radiosurgery used the rigid bony fixation to target spinal tumors (Figs. 16-2 and 16-3).911 Although preliminary results were encouraging, rigid fixation was extremely cumbersome and impractical.

Traditional frame-based stereotactic systems use multiple, overlapping isocenters to achieve some semblance of field shaping (Fig. 16-4). This results in an inhomogeneous distribution of radiation that is often less than ideal for maintaining coverage within a target area and away from critical structures. Such inhomogeneity is particularly disadvantageous when administering very aggressive doses of radiation to non-spherical lesions immediately adjacent to the spinal cord.

The frameless image-guided radiosurgery system, the CyberKnife, developed by Accuray Inc. (Sunnyvale, CA), overcomes many of the limitations encountered by earlier systems. Rather than use an external fixation device, targeting is based on internal radiographic features such as skeletal anatomy or implanted fiducials. The system also allows multiple images to be acquired during treatment to track and compensate for any changes in target location. Finally, instead of using fixed isocenters, the CyberKnife is capable of distributing doses in a conformal and homogeneous manner that enables the treatment of irregularly shaped lesions with unmatched precision (Fig. 16-5).

TECHNICAL OVERVIEW OF THE CYBERKNIFE SYSTEM FOR SPINAL RADIOSURGERY

The CyberKnife radiosurgery system (Fig. 16-6) consists of a lightweight 6-MV linear accelerator mounted on a computer-controlled robotic arm. The robot receives information regarding target location via an x-ray based imaging feedback system. This system consists of two orthogonally aligned x-ray cameras that acquire radiographs of targeting landmarks during treatment. The x-ray cameras are at fixed positions within the treatment room thereby providing a stationary frame of reference for spinal localization. Once the images are referenced within the imaging system’s coordinate frame, the position of the lesion is known. Targeting is based on the assumption of a fixed relationship between the lesion and the spine. The radiographs acquired by the real-time imaging system are compared to digitally reconstructed radiographs (DRRs) derived from computed tomography (CT) scans obtained during pretreatment planning (Fig. 16-7). The CyberKnife software accounts for both translation and rotation of the patient’s anatomy by changing the position of the DRR until an exact match of the x-ray image and DRR is achieved.12 This algorithm eliminates the need to fix the orientation of the patient during treatment. Once the location of the spine is determined, the coordinates are relayed to the robotic arm, which controls the targeting of the linear accelerator (LINAC). This concept enables the system to detect and adjust to changes in target position in less than 1 second with an accuracy approaching ±0.5 mm.13,14

The CyberKnife targeting algorithm is designed by positioning the lesion within the center of an 80-cm3 sphere that is defined with respect to the patient’s anatomy. On the surface of the sphere, there are 100 equally spaced points called nodes. At each node the robot defines 12 beams of radiation that intersect various portions of the tumor volume. The robotic arm stops at each node where radiation beams of a specific prescribed dose are administered. Treatment plans are created from a subset of the entire constellation of radiation beams that concentrates dose to the target area while minimizing dose to the adjacent normal tissue. In practice, not all nodes are available because of objects within the treatment room that obstruct the path of certain beams or prevent the robotic arm from positioning the LINAC at a particular node. At least 50 nodes and approximately 95–200 beams are used during treatment. Since the robot can aim a beam virtually anywhere within the tumor volume, highly conformal treatment plans can be created for complex tumor morphologies in a non-isocentric manner (Fig. 16-8).12

TREATMENT PROCEDURE

SPINAL IMMOBILIZATION

Special immobilization devices are used to provide comfort and to help restrict patient movement during pretreatment scanning and treatment delivery. For the head and cervical spine, a custom-fitted Aquaplast mask (WFR/Aquaplast Corp, Wyckoff, NJ) is constructed; the mask consists of a thermoplastic mesh that conforms to the patient’s face (Fig. 16-9). The patient’s head and neck rest on a radiolucent support pillow while the mask is fastened to the treatment table. For thoracic and lumbar spine lesions, a custom vacuum cradle is form fitted to support the body. The patient is placed supine on the vacuum cradle, which is secured to the body using large Velcro straps fastened to the treatment table (Fig. 16-10).

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