Frame-Based vs Frameless Gamma Knife: Pinned Frame, Thermoplastic Mask and Hypofractionation

Frame-based Gamma Knife pins a rigid metal frame to the skull to hold the head still for a single treatment, while frameless Icon-generation units use a custom thermoplastic mask and cone-beam CT instead, which allows the dose to be split over a few sessions. Both deliver the same focused cobalt-60 radiosurgery to an accuracy of under about 0.5 mm; the difference is how your head is held still and whether the treatment happens in one day or a few¹.

When I was preparing for my acoustic neuroma treatment, the frame was the part I dreaded most, and nobody mentioned that a frameless option even existed until I asked. It turned out my centre used the pinned frame, and my target suited a single session, so that was that. But the question I kept circling was a fair one: if a mask exists, why would anyone still choose to have their skull pinned into a frame? This is the honest comparison I wish I had been handed. For the physics that both approaches share, read how Gamma Knife works, and for the wider overview of the treatment, start with Gamma Knife radiosurgery.

What is the difference between frame-based and frameless radiosurgery?

The difference is entirely in how the head is immobilised: a rigid frame pinned to the skull versus a moulded thermoplastic mask, with everything about the radiation itself staying the same. In both cases the machine converges many individually weak beams of cobalt-60 gamma radiation so they meet at the target, delivering a high dose there while sparing the tissue around it, to an accuracy of under about 0.5 mm².

The traditional method is frame-based. A lightweight metal frame is fixed to the skull with four pins so that your head cannot move relative to the machine, which is what makes precise targeting possible. The newer method, available on Leksell Icon-generation units, replaces the frame with a custom thermoplastic mask and uses cone-beam CT imaging to check your position, with a camera monitoring tiny movements throughout³. The radiation source, the beam count and the convergence are identical; only the immobilisation and, as a result, the number of possible sessions differ.

How does the pinned head frame work?

The pinned frame is a rigid metal ring fixed to the skull with four pins after four small injections of local anaesthetic, and it locks your head in a fixed relationship with the machine for a single-session treatment. The anaesthetic numbs the four points first; what you feel as the pins tighten is pressure rather than pain, though brief soreness at the pin sites afterwards is a recognised, uncommon effect¹.

The frame does two jobs at once. It holds your head completely still, and it also acts as the coordinate system the planning team uses to locate the target in three dimensions. Because it is fixed for the whole day, the frame commits you to a single session: imaging, planning and treatment all happen while it stays on, and it is removed the same day. That single-day, day-case flow is the classic Gamma Knife experience. My own frame stayed on for the long middle wait while the team planned the dose, which was the strangest hour of the day, sitting still with a frame on my head while people I could not see worked out where the beams would go. For the honest, first-hand version of that day, see what the Gamma Knife frame feels like and the day of Gamma Knife hour by hour.

How does the frameless thermoplastic mask work?

The frameless system holds your head still with a custom thermoplastic mask moulded to your face, then uses cone-beam CT to confirm your position and an infrared camera to monitor small movements during treatment, so no pins go into the skull. The mask is warmed, softened and pressed to your features so it sets to your exact shape; there are no injections and nothing sharp, which is the part I would have found reassuring to know³.

Because the mask can be taken off and put back on with the same accuracy day after day, it removes the single-day constraint of the frame. The cone-beam CT scanner built into the Icon unit takes a fresh image at each session and adjusts the setup to match the plan, and the motion-monitoring system pauses the beam if you move beyond a small threshold. The trade-off is that this precision depends on imaging and monitoring rather than on rigid fixation, whereas the frame delivers immobilisation directly. Some people find the enclosed mask claustrophobic even though it does not hurt, which is worth knowing in advance. For how the platforms compare more broadly, including the frameless robotic alternative, see Gamma Knife versus CyberKnife.

Single session versus hypofractionation

A frame commits you to a single high-dose session, whereas the mask lets the dose be split into a few sessions, usually 2 to 5, which is called hypofractionation or stereotactic radiotherapy. A single high-dose treatment is termed radiosurgery; when the same idea is divided into a few fractions it is termed stereotactic radiotherapy, and dividing it can spare healthy tissue near the target⁴.

The reason this matters is that not every target suits one big dose. Single-session radiosurgery is typically used for targets around 3 to 3.5 cm or smaller. Larger ones, or targets sitting right next to critical structures such as the optic nerves or brainstem, may be treated with the dose staged or split over a few sessions to protect what is nearby, and that is far more practical without a frame pinned in place¹. So the mask is not simply a comfier frame: it unlocks a different way of delivering the dose. To understand which targets qualify for which approach, see am I a candidate for Gamma Knife and Gamma Knife planning and dose.

The trade-offs: comfort, accuracy and which suits which target

The core trade-off is rigid, self-contained one-day precision from the frame versus repeatable, pinless, fractionable positioning from the mask, and the target decides which wins. Both hold the beams to under about 0.5 mm, so this is not a question of one being accurate and the other not⁵. It is a question of how that accuracy is achieved and what it lets you do.

The frame’s strengths are that it immobilises directly, needs no imaging or monitoring during treatment to stay accurate, and finishes in a single day. Its costs are the four pins and injections, the brief pin-site soreness, and the fact that it commits you to one session. The mask’s strengths are no pins, no injections, comfort for people who dislike the frame, and the ability to fractionate for larger or delicate targets. Its costs are that it depends on cone-beam CT and motion monitoring to hold precision, that not every centre has an Icon-generation unit, and that treatment may span several visits.

One point worth being clear about: you do not usually get to pick. The choice is clinical, made by the team from your scans, based on the size, shape and position of the target and whether the dose needs splitting. It is a good question to ask at consultation, and it is on my list in questions to ask before Gamma Knife. For what recovery looks like afterwards, whichever route you take, see Gamma Knife recovery, and for how the pins are managed on the day, the Gamma Knife frame.