Radiosurgery is the term used to refer to a class of radiation techniques that are spatially very precise and enable the dose to very rapidly fall-off in the immediately surrounding anatomy. This approach makes it possible to administer aggressive, and even ablative, doses of radiation without damaging adjacent normal tissue. To achieve the above objectives, radiosurgery is delivered in a small number (typically 1 to 5) of daily treatments.

Radiosurgery was conceived in 1951 by Swedish neurosurgeon Lars Leksell as a non-invasive method for making precise lesions in the brain. The method that Lars Leksell developed, Gamma Knife radiosurgery, used a metal frame that was screwed into the patient's skull to both immobilize (hold completely still) the head and accurately target radiation within the brain. Although this device was quite effective for brain radiosurgery, it was not practical to attach external frames to other parts of the body. As a result, until recently, radiosurgery has generally been restricted to treatment of intracranial (inside the skull) disorders. This shortcoming of first generation radiosurgical instrumentation is particularly unfortunate because many of the same lesions that have been so effectively treated with radiosurgery in the brain also occur throughout the spine

The first device for performing spinal radiosurgery was developed at the University of Arizona and reported in 1995. This procedure used an external body frame that was surgically attached to the spine and pelvis under general anesthesia. However, the invasiveness of this approach precluded widespread acceptance. It wasn't until the development of the CyberKnife that a more elegant, i.e. less invasive, means of performing accurate spinal radiosurgery became available.

How does CyberKnife spinal radiosurgery work?

The CyberKnife enables the minimally invasive and extremely accurate delivery of aggressive doses of radiation to many spinal lesions. At the heart of this surgical instrument is a miniature linear accelerator, for producing high energy therapeutic x-rays, attached to a robotic arm. While the robot arm moves extensively around a patient, x-ray beams are precisely targeted to the tumor from hundreds of different directions. Perhaps of even greater significance, the CyberKnife eliminates the need for a rigid external frame to be attached to the patient's skeleton. Instead, the CyberKnife uses an x-ray (diagnostic energy as opposed to therapeutic radiation) image-guidance system that identifies the real-time position of the patient's bony anatomy and target throughout treatment delivery. The CyberKnife tracks the position of a spinal tumor with reference to either the skull anatomy (for tumors of the upper neck), or with reference to small stainle ss steel screws called fiducials. These metallic markers are anchored to the bones of either the lower neck, mid or lower back, the cervical, thoracic and lumbar spine, respectively, in a short outpatient procedure. Once in position, these fiducials can be automatically detected by the image-guidance system of the CyberKnife and used to determine the exact position of the tumor. Throughout treatment, the system continually images and identifies these markers, communicating their updated position to the robotic arm so it can precisely re-adjust the aim of the radiation beam to compensate for any small movements made by a cooperative patient. This capacity to accurately target a spinal tumor without rigid external immobilization makes the CyberKnife a unique solution for non-invasively treating many spinal lesions with radiosurgery.

Which spinal tumors are treatable with the CyberKnife?

The CyberKnife has the potential to treat most types of spinal tumors. However, if a lesion is to be considered for CyberKnife radiosurgery, it must be reasonably discrete (distinct from the surrounding normal tissues) on CT or MRI images, and it cannot be too large in size--generally speaking, a volume of 150 cc is the upper limit of what is treatable. Both benign (non-cancerous) and malignant (cancerous or capable of spreading to other sites in the body) spinal tumors can be treated with the CyberKnife. While some types of lesions develop within the substance of the spinal cord itself, these tumors more commonly develop in and around the vertebral column and disrupt neurologic function by compressing the spinal cord. The following lists detail the tumor pathologies that most commonly affect the spine and which can often be treated with the CyberKnife depending on clinical circumstances:

Benign Lesions:

meningioma
schwannoma
neurofibroma
hemangioblastoma
arteriovenous malformations
chondrosarcoma
chordoma
ependymoma
other: giant cell tumor, aneurysmal bone cyst, epidermoid

Malignant Tumors:

spinal metastases from many sources
myeloma
lymphoma
Ewing sarcoma

Spinal Metastases

Metastasis is a process whereby a malignant tumor originating from one part or organ of the body (a "primary" cancer) spreads to another location in the body, usually through the bloodstream. The spinal column (vertebral spine) is one of the most common sites for metastasis. The primary cancers that most frequently spread, or metastasize, to the spine include cancer of the breast, lung, kidney, prostate, and skin (malignant melanoma). Less common metastatic cancers of the spine include thyroid, cervical, and colorectal cancer as well as sarcoma and lymphoma.

Conventional radiation therapy is the standard treatment for painful metastases affecting the spinal column. However, conventional radiation therapy has several disadvantages, including more side-effects within the surrounding normal tissue and a more prolonged course of treatment compared to CyberKnife radiosurgery. Furthermore, some types of metastatic cancers, such as melanoma, sarcoma and renal cancer, respond very poorly to standard radiotherapy, yet are well-controlled after more aggressive CyberKnife treatment.

As a general rule, it is safest and easiest to treat tumors with radiosurgery that have not been previously treated with conventional fractionated radiation therapy. However, CyberKnife technology makes it possible to safely contour the dose of therapeutic x-ray away from the spinal cord and other critical radiosensitive structures. As a result, when using the CyberKnife, it is still possible to get good results, even if a spinal tumor has been previously irradiated. In patients with severe spinal cord compression, or impending vertebral (bony) collapse, standard open surgical decompression followed by internal hardware fixation and fusion is generally indicated.

The primary rationale for treating spinal metastases is the relief of pain and prevention of neurologic deterioration. Prospective and retrospective clinical studies conducted at multiple institutions have now demonstrated that CyberKnife treatment of spinal metastases is quite effective in achieving these two objectives. Meanwhile, patients with metastatic cancer who develop neurologic deficits, such as sensory change, bladder dysfunction or weakness, and/or localized back pain, will experience a substantial improvement if treated within a relatively short time of symptom onset. Pain relief and improvement in neurologic function after radiosurgery generally translates into an improved quality of life. Not surprisingly, the response of a spinal metastasis to CyberKnife radiosurgery will vary depending on the type of primary cancer, the duration of neurologic symptoms, tumor location, and size of the tumor.

Benign Tumors

Meningioma and nerve sheath tumors, such as neurofibroma and schwannoma, are relatively resistant to conventional fractionated radiotherapy. Because of the limited radiation tolerance of the spinal cord and other adjacent normal tissues, the dose of conventional radiation therapy required to control these tumors is often prohibitive. As a result, conventional radiation is used only in patients where surgery is not feasible. Fortunately, spinal radiosurgery appears to offer an important new alternative to surgery and conventional radiation for selected patients with benign lesions.

Unlike metastatic tumors that primarily affect the bones of the spine, meningioma and nerve sheath tumors originate from both the soft tissue coverings of the spinal cord and the attached nerves. Consequently, these tumors are often in intimate contact with the spinal cord. As these lesions slowly enlarge, the spinal cord becomes compressed and its normal function is disrupted. In severe cases, patients will become gradually paraplegic or quadriplegic depending on the exact location of the lesion.

When treating benign spinal tumors with radiosurgery, the primary intermediate objective is to stop all tumor growth. Over the longer term, which oftentimes means many years, these tumors will gradually shrink in size. Preliminary results with CyberKnife radiosurgery for meningioma and schwannoma show excellent control of tumor growth. However, experience with biologically identical tumors that occur in the brain, suggests that neurofibromas which occur in the setting of genetic disorders, like neurofibromatosis, may be more difficult to control with radiosurgery, or for that matter, any other form of current treatment. Importantly, in those patients with benign tumors where significant spinal cord compression on imaging studies is accompanied by severe neurologic dysfunction, CyberKnife radiosurgery is usually NOT a good option. In such cases, any clinical benefit from tumor shrinkage takes too long to be recognized

Other Tumors and Lesions

Primary malignant tumors affecting the spine, such as myeloma, lymphoma, osteosarcoma, and Ewing sarcoma, are generally treated by a combination of chemotherapy and conventional radiation therapy. When there are isolated tumors or tumors that recur (re-grow) after conventional treatment, radiosurgery can be considered and is generally quite effective.

Low-grade, i.e. less malignant, tumors of the bony spine and spinal cord, such as chondrosarcoma, hemangioblastoma and ependymoma, are typically managed by open surgical resection. However, for selected patients with these lesions, CyberKnife radiosurgical ablation represents a very effective treatment option. In particular, patients with the genetic condition von Hippel Lindau's disease, who have a proclivity to develop multiple spinal tumors over their lifetime, can be well-managed with CyberKnife spinal radiosurgery if the tumor is caught sufficiently early in its course. Other miscellaneous and even rarer spinal tumors can sometimes be treated with the CyberKnife radiosurgery depending on clinical circumstances.

What is the CyberKnife patient treatment process?

CyberKnife spinal radiosurgery treatment consists of three distinct components:

CT image acquisition based upon skull bony landmarks or implanted bone fiducials.
Treatment planning.
Treatment delivery.

Patient set-up

For lesions located in the neck or cervical spine, patients are first fitted with a non-invasive molded facemask that stabilizes the head and neck on a radiographically transparent headrest. For lesions of the thoracic or lumbar spinef or sacrum, patients must first undergo the placement of tiny markers into their spine that will allow the radiation beam to be focused directly at the lesion. This technique involves a surgical procedure that is usually performed in the operating room. Either general or local anesthesia will be used. The surgeon will place the tiny markers around the lesion through small punctures in the skin. Usually, between four and six markers are used. The procedure is usually performed as an outpatient with minimal pain associated with it.

The patient will then return for imaging. Computed tomographic (CT) images are acquired using a standard CT scanner. Sometimes, an IV is started to inject contrast for the physicians to better see the lesion in question. For patients with allergies to IV contrast who cannot be given IV contrast or for other reasons, non-enhanced CT imaging is performed. The CT scan is usually performed in a normal supine (lying on one’s back) position. For cervical lesions, the facemask is placed to immobilize the head and neck.

Treatment Planning

After the CT scan is performed, a team comprised of a surgeon, a radiation oncologist, and a radiation physicist outlines the lesion in question. Normal structures, such as the spinal cord or kidneys, are also identified. The team creates a treatment plan that will allow the delivery of a large amount of radiation to the lesion in question, while limiting the amount of radiation delivered to normal, healthy structures.

Treatment Delivery

The patient will then return for the treatment itself. Treatments usually last between one to two hours. They are performed in an outpatient setting without any form of sedation. Patients who normally have pain while lying on their back will be asked to take their pain medicines prior to the procedure. The patient lies on a comfortable table known as a "couch." For cervical lesions, the facemask is placed to immobilize the head. For other lesions, because the fiducials or markers are already inside the body, no such immobilization is necessary.

The patient is observed throughout the treatment by closed circuit television. The patient may wave their hand or speak if they would like to temporarily halt the treatment. The patient has the opportunity to pause the treatment at any time if they so desire. There are no direct side-effects of the treatment itself. Occasionally, patients receiving treatment to lower back lesions may experience mild, transient nausea. This is because the radiation passes through the intestines and might irritate the intestines. In these cases, patients might be given anti-nausea medicine prior to the treatment.

After the treatment, the patient is able to go home immediately. There is no recovery time. Occasionally, treatments are fractionated (staged radiosurgery) meaning that they are separated over several days to allow a larger dose of radiation to be given. The decision to fractionate a treatment is determined by the CyberKnife Team on a case-by-case basis.