AANS Neurosurgeon : Features

Volume 20, Number 3, 2011

Stereotactic Radiosurgery in the Management of Spinal Lesions: An Introduction

Carryn Anderson, MD

Stereotactic radiosurgery (SRS) is emerging as a therapeutic option for select patients with single-level metastatic disease of the spine. There are a number of machines on the market that can be used to deliver spinal SRS (e.g., TomoTherapy, CyberKnife, Novalis, TrueBeam), but any linear accelerator that can deliver intensity modulated radiation therapy (IMRT) as image-guided RT (IGRT) can be used to deliver treatment. The critical issue for this emerging modality is the experience level of the neurosurgeon and radiation oncologist, the support of physicists who can perform proper quality assurance, and the inclusion of all members of the multidisciplinary oncology team to decide who is an appropriate candidate.

Abbreviations
EBRT = external beam radiation therapy
IGRT = image-guided radiation therapy
IMRT = intensity-modulated radiation therapy
SBRT = stereotactic body radiation therapy

Usually, spinal SRS is used to deliver a high dose of radiation in a single treatment (also called a fraction). However, sometimes the total dose is divided between two to five fractions to spare toxicity to surrounding normal tissues. The dose delivered varies from 6-10 Gy, which should provide excellent palliation of pain but may only provide temporary relief; to 14-24 Gy, which may be ablative and provide long-term pain relief and local control, at the risk of vertebral fracture or spinal cord/nerve damage. When delivered in more than five fractions, the treatment is referred to as stereotactic body radiation therapy (SBRT)

Planning Treatment
Treatment planning involves a simulation, where the radiation oncology team creates a specialized immobilization device fitted for the patient. The patient is then imaged with CT, and sometimes MRI and PET, while lying in the device. Alternatively, diagnostic MR and PET images can be brought into the treatment planning computer. These images are registered with the simulation CT and the design process begins.

A radiation oncologist and neurosurgeon define the target within all segments of the involved vertebra. The spinal cord and other adjacent normal tissues (i.e. esophagus, great vessels, lung) are also contoured. The radiation oncologist defines the dose to the target as well as limitations to the normal tissues. A dosimetrist and physicist then work together, designing a plan that best meets the defined criteria. The neurosurgeon and radiation oncologist evaluate and approve the final plan (Figure 1) Next, the physics team performs several measurements and specialized quality assurance to confirm the dose distribution will be delivered as expected. Once this is completed, the patient is brought back for a practice run, or “verification,” to confirm proper positioning and feasibility of delivery. The total time between the initial simulation and ultimate delivery of the treatment is approximately a week, but this varies by institution. Delivery of the treatment may require the patient to be in the immobilization device for an hour or more, so poor performance status, severe pain and claustrophobia can be contraindications for this procedure.

Figure 1. Multiple beam angles are used to deposit therapeutic radiation doses to the vertebral body while sparing the spinal cord (white contour on right).

Acute side effects from single-fraction spinal radiosurgery are usually mild and self-limited. Depending on where the radiation beams enter or exit, they could include odynophagia, dysphagia, cough, nausea, loose stools, skin reaction, myalgias, anorexia and fatigue. Late side effects could occur months to years after the treatment and include fracture of the vertebral body, fistula, perforation, neuropathy, and paralysis (exceedingly rare if dose constraints are properly applied). Whenever radiation is delivered, there is a rare risk of radiation-induced malignancy within the treatment field; the timing of development is approximately 10 years later. This is mainly a concern when treating patients with non-malignant pathology or long life-expectancy.

Single-fraction radiation (8 Gy) has proven to provide equivalent pain relief for bone metastases compared to an extended course of radiation (30 Gy in 10 fractions) in breast and prostate cancer.(1) Escalation of dose to 24 Gy in a single fraction has led to 90 percent local control for tumor histologies traditionally thought to be radiation resistant (i.e. renal cell and melanoma). (2, 3) Radiobiological studies are suggesting that the histopathology of such radiation doses results in a substantial change in the tumor microenvironment not seen at conventional doses. (4-6)

Experienced centers have evaluated the cost-effectiveness of this emerging modality compared to traditional large-field external beam radiation (EBRT). While EBRT is the cheaper option, patients may experience increased acute side effects (i.e. dysphagia from esophageal dose or nausea/vomiting from bowel dose) or increased risk for spinal hardware failure compared to SRS (see dosimetric comparison of the two techniques in (Figure 2). (7) (8)

Figure 2. Dosimetric comparison of conventional external beam radiation (left) delivered to multiple vertebral levels with beams in the anterior/posterior plane versus stereotactic body radiation therapy (right) delivered to a single vertebral level with 19 beam angles. Colorwash is displaying percentage of the prescription dose (hot pink 110 percent, red 100 percent, light blue 50 percent, purple 20 percent). Spinal cord is contoured in white.

Conclusion
SRS, as a part of the multidisciplinary approach to managing spinal metastases, is altering the surgical approach to this disease. Where the gold standard for a spinal cord compression per the Patchell trial would be an anterior decompression followed by 30 Gy in 10 fractions with EBRT (9), experienced multidisciplinary teams are now performing decompressive surgery via a posterolateral approach to remove a tumor that is abutting the cord and then performing ablative SRS. (10) Such an approach has resulted in shorter hospitalizations and fewer post-operative complications while maintaining excellent local control.

As this emerging technique is further explored, it is paramount that it is performed at a center for neuro-oncology excellence, where multidisciplinary teams can determine which patients are appropriate candidates and experienced radiation oncology teams can perform this technique safely. For further discussion of current literature, controversy, future directions and issues pertinent to spinal SRS in the post-operative setting, the reader is directed to Sahgal, et al. Journal of Neurosurgery: Spine, April 2011.(11)

Carryn Anderson, MD, is an assistant professor in Radiation Oncology who specializes in spine and brain radiosurgery, CNS, and head and neck cancer at the University of Iowa Hospitals and Clinics in Iowa City, Iowa. The author reported no conflicts for disclosure.