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Stereotactic radiosurgery (SRS)
Radiosurgery delivers a high dose in a single session (or up to 5 fractions) to a small, defined lesion with a steep dose fall-off around it — it suits a few brain metastases, vestibular schwannoma, small meningioma, AVM under 3 cm and trigeminal neuralgia, and spares the whole brain from radiation.
Why it matters in the exam
For brain metastases, SRS alone is preferred to adding whole-brain radiotherapy for 1 to 4 metastases: the N0574 trial showed less cognitive decline with SRS alone, at the price of more intracranial recurrence that requires close MRI follow-up; today SRS is also offered to patients with more metastases when the total volume is small. The dose is set by size — per RTOG 90-05: up to 2 cm — 24 Gy, 2.1 to 3 cm — 18 Gy, 3.1 to 4 cm — 15 Gy. After resection of a metastasis the surgical cavity is irradiated to reduce local recurrence. When whole-brain radiotherapy is given, memantine is added and the hippocampus is spared to preserve memory.
Dose limits in sensitive tissues set safety: the visual system (optic nerves and chiasm) tolerates up to about 8 Gy in a single session, so a lesion abutting the chiasm is treated in fractions. In vestibular schwannoma under 3 cm tumour control is very high, and hearing is preserved in some patients; in AVM obliteration occurs after 2 to 3 years. Radiation necrosis appears months to years after treatment and looks like tumour progression: perfusion MRI shows low blood volume in necrosis and high in tumour; symptomatic necrosis is treated with steroids, bevacizumab or resection. The devices: Gamma Knife (cobalt-60 sources), linear accelerator (LINAC) and CyberKnife.
Key points
- High dose in a single session with a steep fall-off at the margin.
- Metastases: SRS alone — less cognitive harm.
- Dose by size: 24, 18, 15 Gy.
- Chiasm — up to about 8 Gy in a single session.
- Necrosis versus progression — perfusion MRI.
Summary table
| Indication | Accepted limit | What to expect | Note |
|---|---|---|---|
| Brain metastases | 1 to 4, and more when volume is small | High local control | N0574: less cognitive harm than whole-brain radiotherapy |
| Vestibular schwannoma | Under 3 cm | Growth arrest | Partial hearing preservation |
| AVM | Under 3 cm, eloquent or deep area | Obliteration after 2 to 3 years | Bleeding risk until obliteration |
| Meningioma | Small or residual | Tumour control | Also after subtotal removal |
| Trigeminal neuralgia | Refractory or unfit for surgery | Pain relief after weeks | Facial numbness possible |
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Sources
- ASTRO Clinical Practice Guideline: Radiation Therapy for Brain Metastases, 2022 — SRS versus whole-brain radiotherapy
- RTOG 90-05 dose-escalation study for single-fraction radiosurgery, 2000 — Maximum tolerated dose by size
- NCCTG N0574 (Brown et al., JAMA), 2016 — Cognitive outcome with SRS alone versus SRS plus WBRT
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.