When patients hear they need radiation therapy for a brain condition, the natural response is apprehension. Radiation is invisible, and for most people, it conjures images of something dangerous and unpredictable. However, stereotactic radiosurgery (SRS) is among the most precise and targeted treatments available in modern medicine. This article explores three primary technologies used to deliver SRS, including Gamma Knife, CyberKnife, and standard linear accelerator (LINAC)-based treatments, and explains how each approach serves different clinical needs.
What is Stereotactic Radiosurgery?
Despite the name, stereotactic radiosurgery doesn’t involve any incision. It’s a non-invasive procedure that delivers highly focused radiation to specific targets in the brain. The “stereotactic” part refers to the three-dimensional coordinate system used to pinpoint the exact location of the treatment area, allowing clinicians to deliver therapeutic doses with sub-millimeter accuracy while minimizing exposure to surrounding healthy tissue.
SRS is commonly used to treat brain metastases (cancer that has spread to the brain from elsewhere in the body), benign tumors, arteriovenous malformations, and functional disorders like trigeminal neuralgia. Each technology brings different strengths to these applications.
Gamma Knife: Precision Through Radioactive Sources
Insights from Joe Koh, medical physicist with CAMP
Gamma Knife has been used to treat brain conditions for approximately 80 years, making it one of the oldest and most established radiosurgery platforms. Unlike linear accelerators that generate radiation electronically, Gamma Knife uses Cobalt-60 radioactive sources, or small cylindrical “pills”, arranged in a helmet-like configuration around the patient’s head.
“The way I like to picture it,” explains Koh, “is that when patients go into the machine, it looks like their head is getting stuck into a pot. On the other side of that pot are holes, like a colander, that all point to one spot.”
Each of those holes contains a radioactive source, and when they’re all open, the beams converge at a single point to create a precise sphere of therapeutic dose.
Key Advantages
Gamma Knife’s primary strength lies in its dose fall-off characteristics. Since Cobalt-60 produces radiation at very specific, predictable energy levels (unlike the broader spectrum from linear accelerators), the transition from therapeutic dose to minimal dose is extremely sharp.
“The radiation is very targeted, very local, and it falls off very quickly,” Koh notes. “With other systems, the fall-off is broader, so you run the risk of more normal tissue damage.”
This precision makes Gamma Knife particularly well-suited for treating small, critically located lesions where sparing surrounding brain tissue is paramount. Conditions like trigeminal neuralgia, a painful nerve disorder treated by delivering very high doses to a tiny target, are considered ideal Gamma Knife cases.
The system’s relative simplicity also translates to ease of use.
“The planning system is designed for doctors to use exclusively,” Koh explains. “Neurosurgeons who do nothing with radiation are able to come in and do the planning.” This accessibility stems from Gamma Knife’s limited but well-defined capabilities – it does one thing exceptionally well.
Practical Considerations
Gamma Knife’s radioactive sources present unique operational challenges. Since the GammaKnife utilized natural radioactive isotopes which decay over time, the radiation is always present but is stored securely in shielding while not in use. The radiation has a half-life of approximately five years, so a facility that installs new sources will see treatment times double within about five years.
“Right now is that sweet spot where it’s fast enough that we can use it efficiently,” Koh says of his facility’s two-year-old sources. “But in two and a half years, every treatment is going to take twice as long.”
Treatment times currently range from about 10 minutes for small, straightforward cases to 30-40 minutes for more complex ones. Multiple lesions can be treated in the same session, with the system repositioning the patient between targets. However, large treatment volumes are generally not suitable for Gamma Knife because delivery times become impractical. A large hemisphere treatment, for example, could take six hours.
The security requirements around radioactive sources also limit Gamma Knife’s availability.
“The sources themselves could be used for inappropriate purposes,” Koh explains. “You have to get federal background checks and go way up the pecking order to even access it. In this entire hospital, only two of us can get in the room to use it.”
This infrastructure requirement means Gamma Knife systems are typically found only at larger medical centers.
Immobilization Options
Gamma Knife offers two approaches to maintaining patient precision. The first is a thermoplastic mask, which is a heated piece of plastic that molds to the patient’s face and becomes rigid as it cools. While reasonably accurate, it allows some movement. The second option is a rigid frame that attaches directly to the skull using four small screws.
“It’s very graphic,” Koh acknowledges. “Patients usually come out with little marks on their foreheads. But as soon as it’s on, you’re locked in.” Frame-based immobilization is used for cases requiring the highest precision, such as trigeminal neuralgia treatments.
CyberKnife: Robotic Precision and Real-Time Adaptability
Insights from Kiernan McCullough, Chief of Radiation Oncology Physics and medical physicist with CAMP
CyberKnife takes a fundamentally different approach to delivering stereotactic radiosurgery. Rather than fixed radiation sources like Gamma Knife or a gantry-mounted linear accelerator like HyperArc, CyberKnife mounts a compact linear accelerator on a robotic arm. What makes that robotic arm remarkable is that it’s the same technology used on automobile manufacturing lines, specifically the KUKA robot. The same machine that welds car frames with extraordinary repeatability has been adapted here to point a radiation source at a target inside a human body with sub-millimeter consistency.
Patients are positioned on a table at the foot of the robot, which articulates through six joints to deliver radiation from as many as 100 to 200 different beam angles over the course of a single treatment. Two ceiling-mounted x-ray units image the patient every 15 to 20 seconds throughout the session, continuously comparing what they see to the original CT scan that was used for planning.
What makes CyberKnife genuinely distinctive, though, is what it does with that imaging data.
“CyberKnife will actually correct for any of those misalignments,” McCullough explains.
Most other systems monitor patient position and alert the care team if movement exceeds tolerance, but they don’t update where the beam is going. CyberKnife is one of the only widely available commercial platforms that actively adjusts its targeting in real time.
This has meaningful clinical consequences. For a treatment where the target can shift slightly as surrounding anatomy moves, that continuous correction gives clinicians a high degree of confidence in dose delivery. Research has even suggested that patients who received CyberKnife treatment for prostate cancer experienced fewer urethral complications compared to those treated on a linear accelerator, a difference McCullough attributes in part to that updated tracking.
The system can anchor its tracking to several different surrogates depending on the treatment site. For brain treatments, it locks onto the patient’s skull. For spine treatments, the vertebrae serve as the reference. For tumors elsewhere in the body, a small metal fiducial can be implanted near the target and used as a surrogate, allowing the system to follow the tumor itself even as the patient breathes normally. In some lung cases, the tumor itself can be tracked directly within the breathing cycle.
A Different Kind of Versatility
Unlike Gamma Knife, which is designed exclusively for the brain, CyberKnife can treat areas throughout the body, including spine lesions, lung tumors, liver tumors, kidney tumors, and more. The ability to treat patients while they breathe normally, rather than requiring breath-holding techniques, makes it well-suited for thoracic and abdominal targets where a fiducial can be placed.
However, there are limits to that versatility. CyberKnife cannot acquire a full 3D image during treatment, which matters in areas like the pancreas where the bowel can shift position relative to the original plan.
“It’s not ideal because you can’t take a complete picture of it,” McCullough notes, “but for well-contained targets where a fiducial can be anchored, the real-time tracking more than compensates.”
At the sub-mm scale, CyberKnife performs comparably to Gamma Knife, making it a strong option for functional radiosurgery cases like trigeminal neuralgia where extremely high doses must be delivered to tiny nerve targets.
Patient Experience and Immobilization
CyberKnife uses a thermoplastic mask to immobilize the head for brain treatments, the same heated plastic that molds to a patient’s face and hardens as it cools. For body treatments, patients lie on immobilization equipment including vacuum bags that conform to their shape, in a setup very similar to what they’d experience on a standard linear accelerator. There are no screws, no rigid frames, and no procedures required before getting on the table.
Treatment sessions run longer than linac-based treatments, typically somewhere between 30 minutes and two and a half hours depending on complexity. A single brain lesion might be finished in about 20 minutes, while a patient with 15 or 16 brain metastases could be on the table for well over two hours. Breaks are allowed for longer sessions, though the full treatment is completed in a single day.
Planning and Workflow
The planning process for CyberKnife is more hands-on than the more automated workflow of HyperArc planning on a standard linear accelerator. Physicists must select collimator sizes, specify the number and direction of beams, determine how many monitor units to deliver, and define which tracking modality to use for each case.
“There’s a little bit more manual effort up front,” McCullough explains. “You have to tell it where you want it to point, what you want it to track off of.”
Also worth noting is how tight the timeline from scan to treatment really is. Once a patient is imaged and the physician contours the target volume, which typically happens within a few hours of the CT scan, the planning team can have a complete plan ready in roughly half a day. From there, the physician reviews and approves the plan before treatment begins, adding only a few hours to the overall process. For emergent cases involving brain metastases, the team works to get treatment done as quickly as possible, with a strong effort made to treat before a patient’s MRI is more than a week old.
No Radioactive Sources, No Reload Cycles
Since CyberKnife uses machine-produced x-rays rather than radioactive isotopes, it shares none of Gamma Knife’s source management requirements. The radiation is generated electronically and can be turned off instantly. There are no federal background checks required to enter the treatment room, no NRC-regulated source inventory to maintain, and no reload procedures where the entire radioactive core has to be replaced on a multi-year cycle at significant time and cost. As McCullough puts it, it’s essentially a light switch.
A Technology at a Crossroads
CyberKnife had its most prominent moment in radiation oncology roughly 15 years ago, when it was one of the only platforms capable of treating moving targets with the precision stereotactic radiosurgery requires. Since then, gantry-mounted linear accelerators have closed much of that gap. Modern LINAC systems, such as the Varian Truebeam, offer their own motion management and tracking capabilities, and the Varian HyperArc planning platform has made complex multi-lesion brain treatments practical on standard LINACs.
That doesn’t make CyberKnife obsolete. Its real-time targeting correction remains among the most sophisticated available on any commercial platform. For programs already built around it, for cases where its tracking approach offers a specific advantage, or for patients whose tumor characteristics make that continuous correction especially valuable, it continues to occupy a meaningful clinical role.
LINAC-Based Stereo: Versatility Meets Automation
Insights from Nicole Bunda-Randall, medical physicist with CAMP
Linear accelerators (LINACs) are the workhorses of radiation oncology departments, capable of treating virtually any part of the body. Historically, their broader beam characteristics made them less ideal for the ultra-precise requirements of brain radiosurgery. That’s changed significantly with technological advances in both hardware and software.
Hardware Evolution: High-Definition MLCs and Cones
The key hardware innovation enabling LINAC-based brain SRS is the development of high-definition multi-leaf collimators (HDMLCs). These are the small metal “leaves” that shape the radiation beam as it exits the machine.
“They went from half a centimeter to a quarter of a centimeter,” explains Bunda-Randall. “Reducing the size of the MLC leaves allows us to conform the dose better, which you really need for the brain. All that normal tissue is so precious.”
For locations that do not have access to HDMLCs, however, physical cones are also a treatment option available. Some facilities may opt to have a standard definition MLC but purchase cone applicators that can be utilized to collimate the beam to sub-millimeter, circular diemensions. A benefit of cone-based treatment is that there is a very sharp dose falloff outside of the treatment area. Since many targets in the brain are spherical, this can be an optimal treatment and can even be used to treat very small targets at high doses. Cones are less beneficial for larger or unusually shaped targets, however there are many larger brain tumors that can be acceptably treated on a standard MLC LINAC without cones, as the larger volume is less critically dependent on the smaller leaves. This is especially beneficial to more remote or rural locations that have this equipment.
Software Revolution: HyperArc Planning
While HDMLCs made LINAC-based brain SRS feasible, the HyperArc software platform made it practical for treating multiple brain metastases. Before HyperArc, planning a treatment for a patient with numerous brain lesions was extraordinarily time-consuming.
“We’ve treated patients with 15 to 20 brain mets at one time,” Bunda-Randall says. “That would have taken days if we could even get a plan. So in the past, doctors would say, ‘Let’s just do whole brain radiation.'”
Whole brain radiation, while faster to plan and deliver, comes with significant cognitive side effects. The ability to target individual lesions while sparing healthy brain tissue represents a meaningful quality-of-life improvement for patients. HyperArc’s automation makes this approach viable by handling the complex calculations and optimizations that previously required days of physicist’s time.
“It creates your couch kicks, your collimator angles – it sets all of that for you,” Bunda-Randall explains. “What used to take three days minimum is now half a day with a very high quality plan.”
Speed and Accessibility
One of HyperArc’s most significant advantages is treatment time. While Gamma Knife and CyberKnife treatments can take hours, Linac-based SRS typically fits into a 15-30 minute treatment slot, comparable to a standard radiation therapy session. This matters enormously for patients who are claustrophobic, have difficulty lying still, or need to travel long distances for treatment.
The faster planning workflow also has clinical implications. Brain metastases can grow quickly, so physicians typically want to treat within a week of the MRI used to identify the planning target.
“With this, we have a lot more flexibility because planning doesn’t take so long,” Bunda-Randall notes. “We can really meet those doctor needs of getting treatment done within a certain amount of time of their last MRI.”
Perhaps most importantly, LINAC-based SRS dramatically expands access to brain radiosurgery. While Gamma Knife and CyberKnife systems are found primarily at major medical centers, linear accelerators exist in radiation oncology departments throughout the country.
“This opens up rural areas to an option they didn’t have before,” Bunda-Randall says. “Not every area has access to a Gamma Knife or CyberKnife, but most areas have access to some sort of Linac.”
When Linac-Based SRS May Not Be Ideal
Despite its advantages, Linac-based SRS has limitations.
“When you’re starting to go sub-mm, Gamma Knife and CyberKnife can nail that stuff,” Bunda-Randall acknowledges. Treatments like trigeminal neuralgia, where extremely high doses are delivered to tiny nerve targets, remain better suited to dedicated radiosurgery platforms.
Additionally, while HDMLCs have made LINAC margins competitive with dedicated radiosurgery systems, Gamma Knife and CyberKnife still offer superior sparing of normal brain tissue in certain scenarios – a consideration that may be particularly important for younger patients.
At the sub-millimeter scale, CyberKnife performs similarly to Gamma Knife, but physical cones on a LINAC-based system can also provide comparable accuracy for trigeminal neuralgia.
Choosing the Right Technology
The “best” stereotactic radiosurgery platform depends on multiple factors: the size and number of lesions, their location relative to critical structures, patient comfort considerations, and what technology is available at a given facility. Both Koh and Bunda-Randall emphasize that in many cases, multiple technologies could achieve good outcomes; the question is which approach optimizes the balance of precision, speed, and patient experience for each individual case.
For patients, the most important takeaway may be that stereotactic radiosurgery – regardless of the delivery system – represents a remarkable advance in treating brain conditions non-invasively. The invisible nature of radiation that makes it seem frightening is the same property that allows it to treat tumors and other conditions without a single incision, often in a single session, with minimal recovery time.
“Beyond the terrifying nature of it, it’s still a big ask,” Koh acknowledges about the patient experience. But with proper communication and support, including music, regular check-ins, and heart rate monitoring, most patients complete their treatments successfully. The technology continues to advance, and what once required whole-brain radiation or invasive surgery can now often be addressed with a precisely targeted outpatient procedure.
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Last updated: February 2026
Clinical Disclaimer
This resource is provided for general educational and informational purposes only and is not intended to constitute medical advice, diagnosis, or treatment, nor to replace the independent clinical judgment of a licensed physician or other qualified healthcare professional. Individual treatment plans, safety precautions, and clinical recommendations vary based on patient-specific factors and clinical context. Readers should consult their own licensed healthcare provider regarding any medical condition or treatment decisions.
