Frameless Single-Fraction Stereotactic Radiosurgery for Spine
Object. The role of stereotactic radiosurgery for the treatment of intracranial lesions is well established. Its use for the treatment of spinal lesions has been limited by the availability of effective target-immobilizing devices. In this study the authors evaluated the CyberKnife Real-Time Image-Guided Radiosurgery System for spinal lesion treatment involving a single-fraction radiosurgical technique.
Methods. This frameless image-guided radiosurgery system uses the coupling of an orthogonal pair of x-ray cameras to a dynamically manipulated robot-mounted linear accelerator possessing six degrees of freedom, which guides the therapy beam to the target without the use of frame-based fixation. Cervical lesions were located and tracked relative to osseous skull landmarks; lower spinal lesions were tracked relative to percutaneously placed gold fiducial bone markers. Fifty-six spinal lesions in 46 consecutive patients were treated using single-fraction radiosurgery (26 cervical, 15 thoracic, and 11 lumbar, and four sacral). There were 11 benign and 45 metastatic lesions.
Tumor volume ranged from 0.3 to 168 ml (mean 26.7 ml). Thirty-one lesions had previously received external-beam radiotherapy with maximum spinal cord doses. Dose plans were calculated based on computerized tomography scans acquired using 1.25-mm slices. Tumor dose was maintained at 12 to 18 Gy to the 80% isodose line; spinal cord lesions receiving greater than 8 Gy ranged from 0 to 1.3 ml (mean 0.3 ml). All patients tolerated the procedure in an outpatient setting. No acute radiation-induced toxicity or new neurological deficits occurred during the follow-up period. Axial and radicular pain improved in all patients who were symptomatic prior to treatment.
Conclusions. Spinal stereotactic radiosurgery involving a frameless image-guided system was found to be feasible and safe. The major potential benefits of radiosurgical ablation of spinal lesions are short treatment time in an outpatient setting with rapid recovery and symptomatic response. This procedure offers a successful alternative therapeutic modality for the treatment of a variety of spinal lesions not amenable to open surgical techniques; the intervention can be performed in medically untreatable patients, lesions located in previously irradiated sites, or as an adjunct to surgery.

During the past decade, stereotactic radiosurgery has emerged as a well-established treatment for a number of intracranial neurosurgical disease processes. The role of stereotactic radiosurgery for the treatment of a wide variety of benign and malignant intracranial lesions is also well established. Stereotactic radiosurgery is an effective treatment for brain metastases, either with or without whole-brain radiotherapy, with an 85 to 95% control rate. Its role in the primary treatment of benign lesions, including arteriovenous malformations, pituitary adenomas, acoustic neuromas, and meningiomas is well supported by the literature.

The role of radiotherapy in the treatment of spinal tumors has been well documented. Conventional externalbeam radiotherapy lacks the precision to allow delivery of large doses of radiation near radiosensitive structures such as the spinal cord. The low tolerance of the spinal cord to radiation often limits the treatment dose to a level far below the optimal therapeutic dose. If the radiation dose could be confined more precisely to the treatment volume, as is the case in intracranial radiosurgery, the likelihood of successful tumor control would likely increase at the same time that the risk of spinal cord injury is minimized.

Current frame-based stereotactic radiosurgery devices, equipped with skull fixation devices, do not have the capability to treat lesions below the foramen magnum. Conformal radiotherapy and intensity-modulated radiotherapy are limited by problems with target immobilization; this precludes large single-fraction treatment of spinal lesions. Conventional frame-based devices used for stereotactic radiosurgery in cases of intracranial lesions rely on a rigid frame to immobilize the lesion at a known location in space. The frame acts as a fiducial reference system to provide accurate targeting and delivery of the radiation dose. Intracranial radiosurgery is practical because the lesions are fixed with respect to the cranium, which can be immobilized rigidly in a stereotactic frame. Although spinal lesions also have a fixed relationship to the spine, LINAC-based stereotactic radiosurgery techniques developed for spinal lesions require the direct placement of an invasive rigid external frame system onto the spine.

A new image-guided frameless stereotactic radiosurgery delivery system known as the CyberKnife (Accuray, Inc., Sunnyvale, CA) has been developed that was approved by the United States Food and Drug Administration in 2001 for use throughout the entire spine. The system consists of a lightweight LINAC mounted on a robotic arm. Real-time image tracking allows for the tracking of patient movement with a 1-mm spatial accuracy. The CyberKnife was developed as a noninvasive means to align treatment beams precisely with targets. The system differs from conventional frame-based radiosurgery in three fundamental ways. First, it references the position of the treatment target to internal radiographically determined features such as the skull or implanted fiducials rather than a frame. Second, it uses real-time radiographic imaging to establish the position of the lesion during treatment and then dynamically brings the radiation beam into alignment with the observed position of the treatment target. Third, it aims each beam independently, without a fixed isocenter. Intraprocedural changes in patient positioning are compensated for by adaptive beam pointing rather than controlled through rigid immobilization. This allows the patient to be positioned in the treatment room without precise reproduction of the position in the treatment planning study.

The CyberKnife was first developed for treatment of brain tumors at Stanford University. Since 1994, the device has been used at a number of sites around the world to treat a variety of benign and malignant intracranial lesions. As expected, treatment outcomes have closely mirrored those obtained using conventional frame-based radiosurgery. With the ability to treat lesions outside of the skull because of fiducial tracking, a growing interest in using the CyberKnife to treat spinal lesions has emerged.

The purpose of this study was to evaluate feasibility, clinical response, and safety by using the CyberKnife radiosurgical technique to treat spinal lesions. Because of the spatial precision with which the CyberKnife can administer radiation, it is theoretically feasible to administer a tumoricidal radiation dose in a single outpatient treatment. By minimizing the radiation delivered to surrounding healthy tissue, it should also be possible to decrease the rate of complications.