CyberKnife Frameless Single-Fraction Stereotactic Radiosurgery
Object: The role of stereotactic radiosurgery in the treatment of benign intracranial lesions is well established. Its role in the treatment of benign spinal lesions is more limited. Benign spinal lesions should be amenable to radiosurgical treatment similar to their intracranial counterparts. In this study the authors evaluated the effectiveness of the CyberKnife for benign spinal lesions involving a single-fraction radiosurgical technique.
Methods: The CyberKnife is a frameless radiosurgery system in which an orthogonal pair of x-ray cameras is coupled to a dynamically manipulated robot-mounted linear accelerator possessing six degrees of freedom, whereby the therapy beam is guided to the intended target without the use of frame-based fixation. Cervical spine lesions were located and tracked relative to skull osseous landmarks; lower spinal lesions were tracked relative to percutaneously placed fiducial bone markers. Fifteen patients underwent single-fraction radiosurgery (12 cervical, one thoracic, and two lumbar). Histological types included neurofibroma (five cases), paraganglioma (three cases), schwannoma (two cases), meningioma (two cases), spinal chordoma (two cases), and hemangioma (one case).
Radiation dose plans were calculated based on computerized tomography scans acquired using 1.25-mm slices. Planning treatment volume was defined as the radiographic tumor volume with no margin. The tumor dose was maintained at 12 to 20 Gy to the 80% isodose line (mean 16 Gy). Tumor volume ranged from 0.3 to 29.3 ml (mean 6.4 ml). Spinal canal volume receiving more than 8 Gy ranged from 0.0 to 0.9 ml (mean 0.2 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. Pain improved in all patients who were symptomatic prior to treatment. No tumor progression has been documented on follow-up imaging (mean 12 months).
Conclusions: Spinal stereotactic radiosurgery was found to be feasible, safe, and effective for the treatment of benign spinal lesions. Its major potential benefits are the relatively short treatment time in an outpatient setting and the minimal risk of side effects. This new technique offers an alternative therapeutic modality for the treatment of a variety of benign spinal neoplasms in cases in which surgery cannot be performed, in cases with previously irradiated sites, and in cases involving lesions not amenable to open surgical techniques or as an adjunct to surgery.
The role of stereotactic radiosurgery in the treatment of a wide variety of benign and malignant intracranial lesions is well established. Stereotactic radiosurgery is effective in the treatment of benign lesions, including arteriovenous malformations, pituitary adenomas, and acoustic neuromas. In contrast, the role of radiotherapy in the treatment of benign spinal tumors has been limited. Conventional EBRT lacks the precision necessary to allow for the delivery of large doses of radiation near radiosensitive structures such as the spinal cord. It is the spinal cord's low tolerance to radiation that often limits the treatment dose to a level that is 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 therapeutic radiobiological effect for benign lesions should increase at the same time that the risk of spinal cord injury is minimized.
Current frame-based stereotactic radiosurgery devices do not have the capability of treating lesions situated below the foramen magnum. Conformal radiotherapy and intensity-modulated radiation therapy are limited by problems with target immobilization. This limitation of intensity-modulated radiation therapy precludes high-dose single-fraction treatment to spinal lesions. Conventional frame-based devices used in the stereotactic radiosurgical treatment 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. Intracranial radiosurgery is practical because the lesions are fixed with respect to the cranium, which can be immobilized rigidly in a stereotactic frame. Spinal lesions also have a fixed relationship to the spine; however, LINAC-based stereotactic radiosurgery techniques developed for spinal lesions require that an invasive rigid external frame system be directly applied to the spine and therefore are not technically practical.
A unique image-guided frameless stereotactic radiosurgery delivery system, the CyberKnife (Accuray, Inc., Sunnyvale, CA), has been developed for use throughout the entire spine. The system consists of a lightweight LINAC mounted on a robotic arm. Real-time imaging tracking allows for the tracking of patient movement with 1-mm spatial accuracy. The CyberKnife was developed as a noninvasive means to align precisely treatment beams with targets. It differs from conventional frame-based radiosurgery in three fundamental ways. First, it references the position of the treatment target to internal radiographic 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. Changes in patient position during the treatment are compensated for by adaptive beam pointing rather than controlled through rigid immobilization. This allows the patient to be positioned on the treatment couch without precise reproduction of the position in the treatment planning study.
With the ability to treat lesions outside of the skull by using fiducial tracking, a growing interest in the CyberKnife-based treatment of spinal lesions has emerged. Benign spinal lesions not otherwise amenable to open surgical intervention might benefit from radiosurgical therapy similar to intracranial lesions of the same histological type. 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 treatment as has been the case for intracranial lesions.
Object: The role of stereotactic radiosurgery in the treatment of benign intracranial lesions is well established. Its role in the treatment of benign spinal lesions is more limited. Benign spinal lesions should be amenable to radiosurgical treatment similar to their intracranial counterparts. In this study the authors evaluated the effectiveness of the CyberKnife for benign spinal lesions involving a single-fraction radiosurgical technique.
Methods: The CyberKnife is a frameless radiosurgery system in which an orthogonal pair of x-ray cameras is coupled to a dynamically manipulated robot-mounted linear accelerator possessing six degrees of freedom, whereby the therapy beam is guided to the intended target without the use of frame-based fixation. Cervical spine lesions were located and tracked relative to skull osseous landmarks; lower spinal lesions were tracked relative to percutaneously placed fiducial bone markers. Fifteen patients underwent single-fraction radiosurgery (12 cervical, one thoracic, and two lumbar). Histological types included neurofibroma (five cases), paraganglioma (three cases), schwannoma (two cases), meningioma (two cases), spinal chordoma (two cases), and hemangioma (one case).
Radiation dose plans were calculated based on computerized tomography scans acquired using 1.25-mm slices. Planning treatment volume was defined as the radiographic tumor volume with no margin. The tumor dose was maintained at 12 to 20 Gy to the 80% isodose line (mean 16 Gy). Tumor volume ranged from 0.3 to 29.3 ml (mean 6.4 ml). Spinal canal volume receiving more than 8 Gy ranged from 0.0 to 0.9 ml (mean 0.2 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. Pain improved in all patients who were symptomatic prior to treatment. No tumor progression has been documented on follow-up imaging (mean 12 months).
Conclusions: Spinal stereotactic radiosurgery was found to be feasible, safe, and effective for the treatment of benign spinal lesions. Its major potential benefits are the relatively short treatment time in an outpatient setting and the minimal risk of side effects. This new technique offers an alternative therapeutic modality for the treatment of a variety of benign spinal neoplasms in cases in which surgery cannot be performed, in cases with previously irradiated sites, and in cases involving lesions not amenable to open surgical techniques or as an adjunct to surgery.
The role of stereotactic radiosurgery in the treatment of a wide variety of benign and malignant intracranial lesions is well established. Stereotactic radiosurgery is effective in the treatment of benign lesions, including arteriovenous malformations, pituitary adenomas, and acoustic neuromas. In contrast, the role of radiotherapy in the treatment of benign spinal tumors has been limited. Conventional EBRT lacks the precision necessary to allow for the delivery of large doses of radiation near radiosensitive structures such as the spinal cord. It is the spinal cord's low tolerance to radiation that often limits the treatment dose to a level that is 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 therapeutic radiobiological effect for benign lesions should increase at the same time that the risk of spinal cord injury is minimized.
Current frame-based stereotactic radiosurgery devices do not have the capability of treating lesions situated below the foramen magnum. Conformal radiotherapy and intensity-modulated radiation therapy are limited by problems with target immobilization. This limitation of intensity-modulated radiation therapy precludes high-dose single-fraction treatment to spinal lesions. Conventional frame-based devices used in the stereotactic radiosurgical treatment 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. Intracranial radiosurgery is practical because the lesions are fixed with respect to the cranium, which can be immobilized rigidly in a stereotactic frame. Spinal lesions also have a fixed relationship to the spine; however, LINAC-based stereotactic radiosurgery techniques developed for spinal lesions require that an invasive rigid external frame system be directly applied to the spine and therefore are not technically practical.
A unique image-guided frameless stereotactic radiosurgery delivery system, the CyberKnife (Accuray, Inc., Sunnyvale, CA), has been developed for use throughout the entire spine. The system consists of a lightweight LINAC mounted on a robotic arm. Real-time imaging tracking allows for the tracking of patient movement with 1-mm spatial accuracy. The CyberKnife was developed as a noninvasive means to align precisely treatment beams with targets. It differs from conventional frame-based radiosurgery in three fundamental ways. First, it references the position of the treatment target to internal radiographic 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. Changes in patient position during the treatment are compensated for by adaptive beam pointing rather than controlled through rigid immobilization. This allows the patient to be positioned on the treatment couch without precise reproduction of the position in the treatment planning study.
With the ability to treat lesions outside of the skull by using fiducial tracking, a growing interest in the CyberKnife-based treatment of spinal lesions has emerged. Benign spinal lesions not otherwise amenable to open surgical intervention might benefit from radiosurgical therapy similar to intracranial lesions of the same histological type. 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 treatment as has been the case for intracranial lesions.
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