Gamma Knife for Multiple Brain Metastases: How Many Lesions Can It Treat?

Gamma Knife radiosurgery can treat multiple brain metastases in a single session, and the old limit of about four lesions has given way to a modern approach that judges suitability by total tumour volume rather than by counting the spots. For a limited number of metastases it is generally chosen over whole-brain radiotherapy because it spares the untreated brain, and because new metastases can be treated again later with a further focused session¹.

When I was going through my own acoustic neuroma treatment, I met a woman in the waiting area who was on her third Gamma Knife session for metastases from a breast cancer. I had walked in assuming radiosurgery was a one-off, and she gently put me right: she said her team treated whatever showed up on each scan, kept her whole brain in reserve, and that this was normal now, not a sign anything had gone wrong. That conversation is why I wanted to write this piece plainly. If you are new to all of this, start with the pillar, Gamma Knife radiosurgery, and with Gamma Knife for brain metastases for the single-lesion picture.

How many brain metastases can Gamma Knife treat?

There is no longer a single fixed number: older guidance capped radiosurgery at about four metastases per session, but modern high-volume practice safely treats more, guided by the total tumour volume rather than a simple count. The reasoning is that the risks of radiosurgery, chiefly brain swelling and radiation necrosis, track the size of what is treated and the dose delivered, not the number of separate targets². Several small metastases can add up to less treated volume, and therefore less risk, than one large lesion.

This is a genuine shift in practice, not marketing. For years the number four was treated almost as a rule; it now functions more as a historical benchmark. A person with six or eight small, well-seen lesions may be a perfectly reasonable candidate, while someone with a single large one may not be. The decision is made by a multidisciplinary team looking at your imaging, weighing volume, position and your general health, exactly as set out in am I a candidate for Gamma Knife.

Why total tumour volume matters more than the count

Suitability for treating multiple metastases is judged mainly by the total volume of tumour being irradiated, because that volume drives both the achievable dose and the risk of side effects. Single-session radiosurgery generally suits targets of about 3 to 3.5 cm or smaller each, roughly 10 to 15 cc in volume, and the same size logic scales up when several lesions are treated together². The team is effectively asking how much brain tissue receives a meaningful dose in total, not how many dots are on the scan.

This is the part I found genuinely reassuring once it was explained. It means a scan showing “multiple metastases” is not automatically worse news for radiosurgery than a scan showing one. It is the millimetres and the millilitres that count. For the underlying size and candidacy rules, see am I a candidate for Gamma Knife and the honest account of how the dose is planned in Gamma Knife planning and dose.

Gamma Knife versus whole-brain radiotherapy: the evidence shift

For a limited number of metastases, focal radiosurgery is now preferred over whole-brain radiotherapy because it delivers a high dose to each lesion while sparing the rest of the brain, which whole-brain radiotherapy irradiates entirely and which can affect memory and thinking. Whole-brain radiotherapy treats the whole brain over several sessions; radiosurgery is the focal, brain-sparing alternative, and it gives better local control of the lesions it does treat¹. That is the heart of the evidence shift over the past two decades, and it is reflected in national guidance on brain metastases³.

The trade-off is honest and worth stating: because radiosurgery only treats what can be seen at the time, it does not prevent new metastases forming in brain that was never irradiated. Whole-brain radiotherapy, by treating everything, catches microscopic disease that a focused beam cannot. That is the real clinical tension, and it is set out in full in Gamma Knife versus whole-brain radiotherapy. For the choice between focused treatment and surgery for a single dominant lesion, see Gamma Knife versus surgery.

What the local control figures actually mean

One-year local control of each treated metastasis is roughly 70 to 90%, and it depends strongly on the dose delivered to the tumour margin: about 92% at 21 Gy or more, falling to about 72% at 16 to 20 Gy. Control means the treated lesion stops growing⁴. It is not the same as the tumour disappearing, and it says nothing about whether new metastases will appear elsewhere; those are two separate questions.

Read carefully, these numbers tell you why dose matters and why very small lesions, which can take a higher margin dose safely, tend to do well. They also explain why the team plans each lesion individually rather than applying one setting across the whole scan. For how control is tracked over time, and why the target usually stays visible on early scans, see Gamma Knife results and follow-up.

Repeat sessions: why one treatment is often not the end

Because radiosurgery treats only the metastases visible at the time, new ones often appear later in brain that was never irradiated, so repeat sessions are common and are a feature of the brain-sparing approach rather than a sign of failure. Older guidance limited each session, but the ability to come back safely is one reason the approach works: you keep the whole brain available for future focused treatment².

The woman I met in that waiting room had understood this before I did. New metastases on a follow-up scan can feel like the ground giving way, but for many people they simply mean another day-case session, planned around the new targets, with the untreated brain still protected. It is emotionally exhausting all the same, and the reality of waiting between scans is covered honestly in radiosurgery and scanxiety.

Risks when treating multiple lesions

The main risks of treating multiple metastases are the same as for a single one, chiefly brain swelling and radiation necrosis, and because they scale with total treated volume the team keeps that volume in view rather than the number of targets. Radiation necrosis, the most important delayed effect, runs roughly 5 to 25% and is driven by lesion size, over about 20 mm, and dose; it is often manageable, for example with steroids¹. Brain swelling is common and is managed with corticosteroids.

Early effects tend to be milder: fatigue, headache and, for frame-based treatment, brief pin-site soreness. None of this is trivial, but it is planned for. For the full account of what to expect, see Gamma Knife risks and side effects and the calm, first-hand explanation in radiation necrosis, what I learned.