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Stereotactic Radiosurgery Overview
Gamma Knife®
Linear Accelerators (LINAC)
Intensity Modulated Radiation Therapy
CyberKnife® Robotic Radiosurgery System
Stereotactic Radiosurgery Overview
Stereotactic radiosurgery (SRS) differs from conventional radiotherapy in several ways. The efficacy of radiotherapy depends primarily on the greater sensitivity of tumor cells to radiation relative to normal brain tissue. With all forms of standard radiotherapy, the spatial accuracy with which the treatment is focused on the tumor is a secondary concern; normal tissues are protected by administering the radiation dose over multiple sessions (fractions) daily for a period of a few to several weeks. In contrast, radiosurgery, by its very definition, requires much greater targeting accuracy. With SRS, normal tissues are protected by both selectively targeting only the abnormal lesion, and using cross-firing techniques to minimize the exposure of the adjacent anatomy. Since highly destructive doses of radiation are used, any normal structures (such as nerves or sensitive areas of the brain) within the targeted volume are subject to damage as well. Typically, SRS is administered in one to five daily fractions or stages over consecutive days. Nearly all SRS is given on an outpatient basis without the need for anesthesia. Treatment is well tolerated, and rarely interferes with a patient's quality of life. Stereotactic radiosurgery has been used for more than 30 years to treat benign and malignant tumors, vascular malformations, and other disorders of the brain with minimal invasiveness. To date, more than 200,000 patients have been treated worldwide with radiosurgery. The success of SRS is based, to a large extent, on the use of a multidisciplinary approach, which requires close interaction between surgeons, radiation oncologists, medical oncologists, physicists, diagnostic radiologists, radiation therapists, and nurses. This specialized team is responsible for the selection of appropriate patients for SRS, treatment delivery, and long-term follow-up.
Several SRS systems are available for the treatment of patients. The most widely used SRS devices include: cobalt-sourced systems (Gamma Knife®), modified linear accelerators, and the CyberKnife. All of these devices, if properly operated, are capable of delivering the desired radiation dose to a designated target. However, for certain clinical situations, there can be important differences between these devices, which for some patients may have a significant impact on clinical outcome.
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Gamma Knife®
The first radiosurgical device was conceived and developed in the 1950s by Professor Lars Leksell at the Karolinska Institute in Stockholm, Sweden. His work culminated in the development of the Gamma Knife (Elekta Inc), which was used to treat patients beginning in 1968. This device is capable of precisely irradiating small intracranial (inside the skull) target with gamma ray photons. The treated lesion is targeted and the patient's head immobilized (held completely still) through the use of an external metal frame attached to the skull by four screws. A large helmet-shaped device with 201 separate, fixed "holes" or ports allows the radiation emitted by discrete (separate) radioactive cobalt-60 sources to enter the patient's head in small beams that converge on the designated target. The Gamma Knife is designed to treat intracranial targets only.
Advantages of the Gamma Knife include:
- Over 30 years of clinical use with a large number of studies published in the medical literature
- Targeting precision within 2 mm
- Multiple targets in the brain are easily treated during a single treatment sessio
Disadvantages of the Gamma Knife include:
- The basic design limits use to the brain only
- The procedure for radiation targeting requires the placement of a somewhat painful stereotactic head frame
- It can be difficult to treat patients with lesions located in certain areas (e.g. the periphery) of the brain
- It cannot be used for staged radiosurgery (delivering the radiation dose in more than one fraction or treatment session); staged radiosurgery can be particularly beneficial for larger tumors or lesions located near nerves and other sensitive structures
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Linear Accelerators (LINAC)
An alternative to the Gamma Knife was developed in the mid 1980s and utilized the conventional linear accelerators (LINAC) that are commonplace in most large hospitals. By combining a series of small modifications to the radiation delivery mechanism of the LINAC with specialized planning software, it is possible to do many types of brain radiosurgery. There are both dedicated and non-dedicated LINAC-based radiosurgery devices. Dedicated LINAC systems are used solely for radiosurgery treatment. In contrast, non-dedicated systems are the daily workhorses for conventional radiation therapy departments which can also be temporarily modified to perform radiosurgery. Compared to the latter multi-purpose LINAC, dedicated systems tend to be more carefully calibrated for spatial accuracy and optimized for radiosurgical efficiency. Unlike the radioactive cobalt-based Gamma Knife, LINAC -based systems use X-ray beams generated from a linear accelerator. As a result, these devices do not require or generate any radioactive material. Common brand names for modified LINAC include XKnife™ (Radionics™ Inc).
Advantages of Multi-Purpose LINAC Radiosurgical Systems include:
- More commonplace technology in hospital
Disadvantages of Multi-Purpose LINAC Radiosurgical Systems include:
- Less accurate
- Less efficient than dedicated systems, which results in longer treatment time
- Frame-based targeting only works for brain lesions
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Intensity Modulated Radiation Therapy
The recent development of IMRT or Intensity Modulated Radiation Therapy has added another dimension to multi-fraction radiation therapy. These LINAC-based technologies use computer-controlled "beam-shaping" to do a better job of conforming the radiation dose to the shape of the tumor or other lesion. This form of advanced radiation therapy can be utilized at virtually any location in the body. IMRT technology enables a mechanical device (called a multi-leaf collimator) that is typically attached to most modern medical linear accelerators, to dynamically reshape the outlines and intensity of the radiation field during cancer treatment. When combined with sophisticated planning software, IMRT fits the dose of radiation to a target much better than conventional radiation therapy, and thereby minimizes the volume of surrounding normal tissue that is injured by treatment. While it appears that IMRT may produce fewer side-effects than conventional radiation therapy, IMRT is not as spatially precise as radiosurgery. Because of this imprecision, a full course of IMRT treatment is typically administered over multiple treatment sessions (typically 20-30+). Common brand names include X-Knife™ (Radionics™) and Novalis® (Brain Lab).
Advantages of Shaped-Beam systems include:
- The capacity to treat most regions of the body with IMRT
- When coupled to an invasive stereotactic frame, precision targeting for brain tumors that approaches, but does not equal, that of the Gamma Knife or CyberKnife.
- The capacity to more accurately target extracranial (non-brain) tumors than standard radiation therapy
- An ability to deliver fractionated intracranial or extracranial treatmen
Disadvantages of the Shaped Beam systems include:
- The need for an invasive head frame (similar to the Gamma Knife) to assure treatment accuracy when used for brain radiosurgery (single fraction)
- Less treatment accuracy when multiple fractions are used to treat areas of the brain where the use of an invasive head frame is impractical
- A significantly lesser degree of targeting accuracy when treating extracranial tumors compared to brain radiosurgery
- Treatment accuracy is degraded further when the target moves during radiation delivery from either natural breathing or patient movement
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CyberKnife® Robotic Radiosurgery System
The CyberKnife System is an SRS system utilizing contemporary technology that is designed for sub-millimeter accuracy and flexibility. The CyberKnife was designed to address the limitations of frame-based SRS systems and expands the application of radiosurgery to sites outside of the head. It is the only system to incorporate a miniature linear accelerator mounted on a flexible, robotic arm. An image-guidance system that can track target location during treatment also enables the CyberKnife to offer superior targeting accuracy without the need for the invasive head frame. While Gamma Knife and LINAC-based systems can perform radiosurgery in the brain, true radiosurgery for areas outside of the brain is difficult if not impossible to perform with these systems. For more detailed information on the CyberKnife, see CyberKnife Overview.
Advantages of the CyberKnife include:
- No invasive head frame or other rigid immobilization device is required
- The ability to perform radiosurgery (1-5 fractions/stages) on targets throughout the body, not just the brain
- Precise targeting of selected lesions in the brain and body
- A unique ability to provide real time monitoring of the treated target throughout treatment using an advanced image-guidance system
- A unique ability to correct during treatment for limited target motion (e.g. due to small patient movements)
- The capacity to easily perform staged radiosurgery
Disadvantages of the CyberKnife include:
- Some tumors require the placement of very small markers (fiducials) via a needle for the treatment of targets outside of the head
- Compared to other radiosurgical devices, treatment takes longer when multiple tumors are ablated during the same treatment session.
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