What Is Stereotactic Radiosurgery? Focused Radiation Without a Cut

Stereotactic radiosurgery (SRS) is a way of treating a target in the brain with a focused, high dose of radiation instead of an operation: many individually weak beams are aimed so they converge on the target, delivering a high dose there while sparing the tissue around it, with no incision and no knife. Despite the “surgery” in the name, nothing is cut. It is a category of treatment, and Gamma Knife is the best known machine in it¹.

When my acoustic neuroma was found, the first word the consultant used was “radiosurgery”, and I nodded as though I understood it. I did not. I pictured a robot with a scalpel. It took me weeks of reading to grasp that the word describes focused radiation, that several different machines can deliver it, and that the term shifts depending on how the dose is scheduled. This is the plain map I wish I had been handed on day one. For the specific machine I was treated on, see Gamma Knife radiosurgery; for the physics in full, see how Gamma Knife works.

What is stereotactic radiosurgery?

Stereotactic radiosurgery is the delivery of a single, precisely focused high dose of radiation to a defined target, using many beams that each pass harmlessly through the brain and add up to a treatment dose only where they cross. “Stereotactic” means the target is located in three-dimensional coordinates, so the beams can be aimed at it to within a fraction of a millimetre. On Gamma Knife the beams converge to an accuracy of under about half a millimetre, which is what makes it possible to treat a lesion sitting right next to a nerve or a blood vessel¹.

The point that took me longest to accept is that this is not surgery in any ordinary sense. There is no incision, no knife, and no general anaesthetic; you are awake, and most people go home the same day². The word “surgery” survives from the precision of the targeting, not from any cutting. For how that precision is set up on the day, see the Gamma Knife frame and Gamma Knife planning and dose.

Radiosurgery versus stereotactic radiotherapy: the terminology

A single high-dose treatment is called radiosurgery; when the same focused technique is divided into 2 to 5 smaller sessions it is called stereotactic radiotherapy. The machine and the principle are the same. What changes is the schedule: one large dose in a day, or the dose split (fractionated) over a few visits³.

Splitting matters because it lets the team treat larger targets, or targets close to sensitive structures like the optic nerves or brainstem, more gently. A single session typically suits targets around 3 to 3.5 cm or smaller; bigger ones are often staged or fractionated instead⁴. This is also why you will see the same treatment called different things at different centres, which confused me no end until someone drew the distinction out. The frame-based single session and the mask-based split-dose approach are compared in frame-based versus frameless radiosurgery.

The platforms: Gamma Knife, CyberKnife, LINAC and proton

Several different machines deliver stereotactic radiosurgery, and they differ in the radiation they use, whether they need a frame, and whether they can treat the body as well as the brain; the right one depends on the target, not the brand. Here is the high-level shape of them:

• Gamma Knife: uses cobalt-60 gamma rays from a fixed array of sources. Current models use 192 beams (older units used 201), converging to under about half a millimetre. It treats intracranial targets only, usually with a rigid head frame in a single session².
• CyberKnife: a linear accelerator (6 MV X-ray photons) mounted on a robotic arm. It is frameless and can treat the body and spine as well as the brain, often over a few sessions. It delivers a comparable radiation effect to the tissue¹.
• LINAC-based systems (for example Novalis, TrueBeam): use X-rays rather than cobalt-60, treat lesions up to about 3.5 cm, and are more widely available than dedicated Gamma Knife units¹.
• Proton therapy: uses charged particles that deposit most of their energy at a set depth and then stop, which limits the dose beyond the target. It can be delivered stereotactically but is far less widely available.

I spent a long time comparing machines before I understood that my tumour, its size, and its position had already narrowed the sensible options down to a couple. The detailed comparisons live in Gamma Knife versus CyberKnife and Gamma Knife versus LINAC and proton.

How stereotactic radiosurgery differs from whole-brain radiotherapy

Stereotactic radiosurgery is focal and brain-sparing: it concentrates the dose on one or a few defined targets, whereas whole-brain radiotherapy treats the entire brain over several sessions and can affect memory and thinking. For brain metastases in particular, focal SRS is increasingly preferred because it gives better local control while sparing healthy tissue¹.

That difference is not abstract. SRS alone is generally chosen for a limited number of metastases, and modern practice is guided by the total tumour volume rather than a simple count. The trade-off between treating one spot precisely and treating the whole brain is set out in Gamma Knife versus whole-brain radiotherapy.

Who is stereotactic radiosurgery for?

Radiosurgery suits small, well-defined targets that a team can see clearly on imaging and reach without opening the skull, generally around 3 to 3.5 cm or smaller. Gamma Knife is used for lesions of roughly 5 to 40 mm¹. Suitability turns on size, position and type of target, not on preference, and it is decided by a team looking at your scans⁴. The common indications, each covered in its own article, include:

• Acoustic neuroma (vestibular schwannoma): see Gamma Knife for acoustic neuroma.
• Meningioma: see Gamma Knife for meningioma.
• Brain metastases: see Gamma Knife for brain metastases.
• Arteriovenous malformation: see Gamma Knife for AVM.
• Trigeminal neuralgia: see Gamma Knife for trigeminal neuralgia.

I assumed being “non-invasive” made radiosurgery automatically the right choice for me; in fact the team weighed it against surgery and against watching and waiting before recommending it. For that decision, see am I a candidate for Gamma Knife and watch and wait versus Gamma Knife.

What to expect from the result

Radiosurgery works gradually rather than instantly: the target usually stays visible on follow-up scans, and the aim is control, meaning it stops growing, not disappearance. This is the single fact I most wish someone had underlined for me before my scans started. Benign tumours change slowly over 1 to 3 years, and the effect is judged on imaging over time, not on how you feel that afternoon³.

Follow-up is built around this slow timeline, which is why the waiting between scans can feel harder than the treatment itself. For how the result is tracked, see Gamma Knife results and follow-up; and for the honest emotional side of that waiting, see radiosurgery and scanxiety. To bring it all together, the overview lives in the pillar, Gamma Knife radiosurgery.