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What is Gamma Knife Radiosurgery? — How the Technology Works, Explained at KIMS Hyderabad
192 cobalt-60 beams, the Leksell stereotactic frame, sub-0.5 mm precision, 50% isodose prescription, and DNA double-strand-break biology — the complete technical explanation of Gamma Knife radiosurgery at KIMS Neuroscience, Secunderabad, Hyderabad.
- Why 192 beams and not one
- A single radiation beam strong enough to destroy a brain tumour would also destroy all the brain tissue it passes through. Gamma Knife resolves this with 192 beams entering from 192 directions — each delivering only 1/192nd of the total dose to any point it passes through (individually harmless). At the convergence point all 192 beams overlap and the accumulated dose is ablative. The result is an extremely steep dose fall-off at the target boundary — a high dose at the target dropping dramatically within a few millimetres.
- The 50% isodose prescription
- Gamma Knife doses are prescribed to the target margin, defined as the 50% isodose surface. The prescription dose (e.g. 12 Gy for an acoustic neuroma) is delivered to the outer margin; the isocenter (centre of the target) receives twice that dose (24 Gy). This is why the cochlea, some distance from the margin, receives a dose much lower than 12 Gy — allowing hearing preservation — and is what allows 97–99% facial nerve preservation in acoustic neuroma treatment.
- Cobalt-60 — the radiation source
- Each of the 192 beams originates from a capsule of cobalt-60, a radioactive isotope that emits gamma rays as it decays. Cobalt-60 has a half-life of 5.26 years; the sources are replaced every 5–6 years to maintain output. They are contained in a thick concrete-shielded room — no radiation exposure occurs outside the treatment room. The Leksell Gamma Knife is manufactured by Elekta AB of Sweden — the platform used at KIMS.
- Leksell stereotactic frame
- KIMS currently uses the Leksell Vantage Frame — a resin frame that is significantly lighter and more comfortable than the traditional Leksell G Frame (a titanium–aluminium alloy), with the same submillimetre accuracy. It is fitted to the skull at four scalp points under local anaesthetic (not general anaesthesia). It creates a rigid, reproducible coordinate system that translates the MRI coordinates of the target precisely to the machine coordinates of the Gamma Knife — the chain of precision that achieves <0.5 mm targeting accuracy. The frame is removed after treatment without any surgical intervention.
- DNA mechanism vs AVM mechanism
- For tumours, single-session 15–24 Gy causes DNA double-strand breaks that overwhelm the cell's repair machinery, producing mitotic arrest and delayed cell death over days to months — which is why response is seen over weeks to months, not immediately. For AVMs the mechanism is different: progressive endothelial damage and fibrous obliteration over 2–3 years. The AVM's haemorrhage risk persists during that obliteration period — a critical counselling point.
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- 040-44885000 · Send MRI for pre-consultation imaging review · No referral required.
| Fact | Details |
|---|---|
| Core principle | 192 cobalt-60 beams from 192 directions · Each delivers 1/192nd of the dose (harmless individually) · Lethal dose only at convergence point · Extremely steep dose fall-off at target boundary |
| 50% isodose prescription | Prescription dose at target margin = the 50% isodose surface · Isocenter receives twice the prescription dose · Critical structures at the margin receive the prescription dose · Structures beyond receive much less |
| Cobalt-60 sources | 192 capsules · Half-life 5.26 years · Replaced every 5–6 years · Contained in shielded room · No exposure outside treatment room · Leksell Gamma Knife (Elekta AB, Sweden) |
| Leksell frame | KIMS: Leksell Vantage Frame (resin; lighter and more comfortable than the titanium–aluminium alloy G Frame; same submillimetre accuracy) · 4 scalp pins (local anaesthetic) · Rigid coordinate system · Translates MRI coordinates to machine coordinates · Chain of precision achieving <0.5 mm accuracy · Removed after treatment |
| DNA mechanism (tumours) | Single-session 15–24 Gy overwhelms DNA repair machinery · Mitotic arrest · Delayed cell death over days to months · Response seen over weeks to months |
| AVM mechanism (different) | Endothelial damage + fibrous proliferation in vessel walls · Progressive obliteration over 2–3 years · Haemorrhage risk persists during obliteration period · Annual MRI to confirm obliteration |
Not a Knife — A Genuinely Different Category of Treatment
Gamma Knife radiosurgery was invented by Swedish neurosurgeon Lars Leksell in 1968 and has since been used to treat more than one million patients worldwide. It is not a knife. It does not cut. The word “surgery” in its name refers to the precision of the dose delivery — the ability to focus radiation on a target within the brain with the spatial accuracy of surgery. The actual procedure involves no incision, no anaesthesia, and no surgical instruments touching the brain. Understanding exactly how Gamma Knife achieves this precision is the key to understanding why it is a genuinely different category of treatment from both conventional radiotherapy and open surgery.
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Speak directly with the KIMS Gamma Knife team. Bring your MRI for a personalised explanation of whether Gamma Knife is right for your specific condition.
040-44885000Emergency line: 040-46324365Non-Invasive · 192 Converging Beams · No Incision · No General Anaesthesia
Gamma Knife radiosurgery is a non-invasive brain treatment that delivers a precisely targeted high dose of gamma radiation to a brain tumour or abnormality using 192 converging beams, destroying the target while sparing surrounding tissue, requiring no incision, no general anaesthesia, and no hospital stay for most patients.
The Gamma Knife Technology Stack
The six key technical concepts that distinguish Gamma Knife from conventional radiotherapy and from other radiosurgery systems — each explained in detail in the sections below.
192 Converging Beams
Each beam delivers only 1/192nd of the total dose to the tissue it passes through — harmless individually. Only at the convergence point do all 192 beams overlap to deliver the ablative dose. The result is a dose distribution with an extremely steep fall-off.
Cobalt-60 Sources
Each beam originates from a cobalt-60 capsule (half-life 5.26 years, replaced every 5–6 years). The Leksell Gamma Knife is manufactured by Elekta AB of Sweden; the unit sits in a thick concrete shielded room.
Leksell Stereotactic Frame
KIMS uses the Leksell Vantage Frame — a resin frame, lighter and more comfortable than the traditional titanium–aluminium alloy G Frame, with the same submillimetre accuracy — fitted at four scalp points under local anaesthesia. Defines the coordinate system that translates MRI pixels to physical machine coordinates — the chain of precision that achieves <0.5 mm targeting accuracy.
50% Isodose Prescription
Prescription dose is delivered to the target margin (the 50% isodose surface) — the isocenter receives twice that dose. The dose drops to 5–6 Gy only 2 mm outside the margin, sparing the cochlea, optic nerve, and brainstem.
Leksell Icon — Frameless Option
Thermoplastic mask with real-time cone-beam CT verification — for hypofractionated multi-session treatment, frame-anxious patients, or larger targets where sub-millimetre variation matters less than for small targets.
Biological Mechanism
Single-session 15–24 Gy doses overwhelm the cell's DNA repair machinery, producing mitotic arrest and delayed cell death over days to months. For AVMs, the mechanism is endothelial damage and fibrous obliteration over 2–3 years.
The Core Principle — Why 192 Beams and Not One
The problem with a single radiation beam
If you aimed a single beam of radiation strong enough to destroy a tumour at a patient's head, you would destroy the tumour — but you would also destroy all the brain tissue that the beam passes through on the way in, and all the tissue on the way out. A single beam powerful enough to ablate the target would be lethal to the surrounding brain. This is why conventional radiotherapy uses repeated small doses (fractions) spread over weeks — allowing normal tissue to repair between sessions while the cumulative effect destroys tumour cells, which repair less efficiently. But fractionation sacrifices precision: you cannot deliver the focal destructive dose of surgery in 30 sessions.
The Gamma Knife solution — 192 beams from 192 directions
Gamma Knife resolves this with a simple but profound insight: if you use 192 beams of radiation, each entering from a different direction and angle, and direct all 192 beams to converge at a single point — the target — then each individual beam delivers only 1/192nd of the total dose to each point it passes through. No single beam is destructive. But at the convergence point, all 192 beams overlap, and the accumulated dose is high enough to be lethal to tumour cells. The brain beyond the target, traversed by each beam individually but never receiving the full converging dose, is largely spared. The result is a dose distribution with an extremely steep fall-off: the dose is high at the target and drops dramatically within a few millimetres. This steep dose gradient is the defining physical characteristic of Gamma Knife radiosurgery and is what allows it to treat targets within the brain with sub-millimetre precision.
Cobalt-60 — the radiation source
Each of the 192 beams originates from a small capsule of cobalt-60 — a radioactive isotope that emits gamma rays (high-energy photons) as it decays. The 192 cobalt-60 sources are arranged in a precise geometric pattern within the Gamma Knife unit (the Leksell Gamma Knife, manufactured by Elekta AB of Sweden). The gamma rays travel through beam channels that focus each beam toward the defined isocenter (the convergence point). Cobalt-60 has a half-life of approximately 5.26 years, meaning the sources are replaced approximately every 5–6 years to maintain dose accuracy. The Gamma Knife unit requires a thick concrete shielded room to contain the radiation between treatments.
The isocenter — where all beams meet
The isocenter is the point in space where all 192 beams converge. Every calculation in Gamma Knife treatment planning is oriented around placing the isocenter precisely within the target — whether that target is a 5 mm acoustic neuroma, a 2 cm meningioma, or the trigeminal nerve root entry zone. For small, compact targets, a single isocenter is sufficient. For larger or irregularly shaped targets, multiple isocenters are used simultaneously, with the dose to each isocenter adjusted so that the combined dose distribution conforms to the target shape. Modern Gamma Knife systems (Leksell Gamma Knife Icon and Perfexion) can deliver radiation to multiple isocenters automatically, without repositioning the patient between shots.
The Leksell Coordinate Frame — The Foundation of Precision
What the frame does and why it is needed
Achieving sub-millimetre accuracy requires that the target coordinates derived from MRI imaging can be translated with absolute precision into physical coordinates in the treatment room. The Leksell stereotactic frame is the device that creates this translation. The traditional Leksell G Frame is machined from a titanium–aluminium alloy. KIMS currently uses the Leksell Vantage Frame — a resin-based stereotactic frame that is significantly lighter than the traditional metal G Frame, improving patient comfort during the frame fixation, imaging, and treatment session while maintaining the same submillimetre stereotactic accuracy. The frame is fitted around the patient's head at four points (two frontal, two occipital) using small titanium pins that penetrate a few millimetres into the outer table of the skull under local anaesthesia. Once the frame is fixed, it defines a precise coordinate system around the patient's head. MRI images are acquired with the frame attached, and every point within the MRI coordinate system can be translated to a corresponding physical coordinate within the frame. The treatment plan specifies the target in frame coordinates, and the Gamma Knife unit positions the patient so that the isocenter in the machine precisely coincides with the target coordinates. This chain of precision — from MRI pixel to frame coordinate to isocenter position — achieves overall targeting accuracy of less than 0.5 mm.
Frameless Gamma Knife — the mask-based alternative
The latest Leksell Gamma Knife Icon system offers a frameless alternative using a thermoplastic mask and cone-beam CT (CBCT) image guidance. The patient wears a fitted mask (similar to those used in conventional radiotherapy) and real-time CBCT imaging is performed during treatment to continuously verify the patient's position and compensate for any head movement. Frameless Gamma Knife is suitable for certain conditions and is particularly appropriate for: (1) Hypofractionated (multi-session) treatment, which requires multiple visits and for which a rigid frame cannot be reattached with identical precision each time. (2) Patients in whom frame placement is technically challenging or who are very anxious about the frame. (3) Conditions where the treatment plan is inherently less sensitive to sub-millimetre accuracy variations (e.g. larger targets). Frame-based treatment remains the standard for single-session radiosurgery requiring maximum precision, particularly for small targets like acoustic neuromas and the trigeminal nerve root.
Dose Planning — The Physics of Precision
Treatment planning software — how the dose distribution is designed
After the frame is fitted and MRI images are acquired, the physicist and radiation oncologist design the treatment plan using specialised software (Leksell GammaPlan). The target (tumour or abnormality) is delineated on the MRI. Critical adjacent structures (optic nerves, brainstem, cochlea) are also delineated to define dose constraints. The software calculates the dose distribution produced by all 192 beams simultaneously, using hundreds of millions of calculations in seconds. The plan is optimised to deliver the prescribed dose to the target while keeping the dose to all critical structures below safe thresholds. The plan is reviewed and approved by the neurosurgeon, radiation oncologist, and medical physicist before treatment begins.
The 50% isodose line — what prescription dose means
Gamma Knife dose is prescribed to the margin of the target, defined by a chosen isodose line (the surface at which a specified percentage of the maximum dose is received). The convention is to prescribe to the 50% isodose line: if the prescription dose is 12 Gy to the 50% isodose, the maximum dose at the isocenter is 24 Gy, and the outer margin of the target receives 12 Gy. The dose falls from 24 Gy at the centre to 12 Gy at the outer target margin over a few millimetres, then drops steeply: 2 mm outside the target margin, the dose may be only 5–6 Gy. This steep gradient is what allows structures very close to the target (the cochlea next to an acoustic neuroma, the optic nerve next to a cavernous sinus meningioma) to receive only a fraction of the target dose.
Gamma Knife vs Other Treatments — Technical Comparison
The comparison matrix below summarises the technical differences between Gamma Knife, CyberKnife, Linac-based SRS, and open surgery.
System
Radiation Source
Beams
Precision
Head Frame
Sessions
Best For
Evidence Base
Leksell Gamma Knife
Cobalt-60 (192 fixed sources)
192 fixed cobalt-60 sources
Frame-based or mask (Icon)
1 (or hypofractionated)
Small, compact brain targets
Largest in radiosurgery (1M+ patients)
CyberKnife
Linear accelerator (robotic arm)
Single beam, multiple positions
Mask-based (frameless)
1–5
Body SRS, some brain
Growing
Linac-based SRS
Linear accelerator (gantry)
Multiple arcs, single beam
Frame or mask
1–5
Brain and body
Moderate
Open Surgery
None
N/A
Mayfield clamp
1
Large tumours, tissue diagnosis
Extensive
Note: CyberKnife and Linac-based SRS are alternative radiosurgery technologies. The choice between them depends on the specific clinical situation. At KIMS, the radiosurgery team uses the system most appropriate for the individual patient's condition.
How Gamma Knife Destroys Tumour Cells
The biological mechanism — DNA double-strand breaks
Gamma radiation destroys cells by causing double-strand breaks in DNA — breaks in both strands of the DNA double helix simultaneously. Normal cells can repair single-strand DNA breaks; double-strand breaks are much harder to repair and more frequently lead to cell death. At the high doses delivered to the isocenter in a single Gamma Knife fraction (typically 15–24 Gy for brain metastases, compared to 2 Gy per fraction in conventional radiotherapy), tumour cells sustain so many double-strand breaks that they cannot repair them all. They enter a state of mitotic arrest (inability to divide) and subsequently die. This process is not instantaneous: tumour cells die over days, weeks, and months following treatment. This is why Gamma Knife does not produce immediate tumour shrinkage — the tumour often remains the same size or even temporarily swells for weeks to months before beginning to contract.
Vascular effects — how Gamma Knife treats AVMs
For arteriovenous malformations (AVMs), the mechanism of action is different from tumour destruction. The high-dose radiation delivered to the AVM nidus causes endothelial cell damage in the abnormal blood vessels of the AVM, triggering a progressive fibrous obliteration of the abnormal channels over 2–3 years. The AVM gradually “closes off” as the treated vessels are replaced by scar tissue. During this obliteration period, the AVM continues to carry blood and carries its usual bleeding risk — which is why surveillance MRI and angiography at 2–3 years after treatment is essential to confirm complete obliteration.
What the Technology Achieves
The Gamma Knife platform at KIMS is grounded in 50+ years of engineering refinement and a less than 1 million patient evidence base. The following points summarise the most clinically significant technical facts every patient and family should understand.
Invented by Swedish neurosurgeon Lars Leksell in 1968 — used to treat over one million patients worldwide
192 cobalt-60 sources arranged in 8 sectors deliver beams that converge at the isocenter — each beam carries only 1/192nd of the dose along its path
Cobalt-60 half-life of 5.26 years — sources are replaced approximately every 5–6 years to maintain dose accuracy
Overall targeting accuracy <0.5 mm — the chain of precision runs from MRI pixel to Leksell frame coordinate to machine isocenter
Dose drops from 24 Gy at the isocenter to 12 Gy at the target margin (50% isodose), then to ~5–6 Gy only 2 mm beyond — the steep gradient that spares adjacent structures
Single-session prescription doses of 15–24 Gy for brain metastases vs 2 Gy per fraction in conventional radiotherapy — the difference that makes Gamma Knife ablative rather than fractionated
Tumour cells die over days, weeks, and months — Gamma Knife produces no immediate shrinkage and often transient pseudo-progression
AVM obliteration is mediated by endothelial damage and fibrous obliteration over 2–3 years — surveillance MRI and angiography confirm complete obliteration
Related Treatments
Explore related Gamma Knife pages at KIMS Neuroscience:
Frequently Asked Questions
Understand Gamma Knife. See Your MRI. Get an Honest Assessment — at KIMS Hyderabad.
To understand whether Gamma Knife is right for your specific condition, a consultation with the KIMS radiosurgery team includes a review of your MRI, a full explanation of the procedure, and an honest assessment of what Gamma Knife can achieve for your case. Call 040-44885000.
International enquiries: info@kimsneurosciences.com