Gamma Knife
The original radiosurgery machine: about two hundred cobalt-60 sources arranged in a shielded helmet whose beams cross at one point inside the brain, so a metastasis or benign tumour a few millimetres across receives a destructive dose in a single visit while the brain around it is spared.
Overview
Lars Leksell treated the first patient with the Gamma Knife at Karolinska in 1968. The modern Elekta models (Perfexion, Icon and Esprit) hold 192 cobalt-60 sources in eight movable sectors, each able to switch between collimator sizes or block, so the dose to each target is built from many small shots. The patient is fixed either in a frame pinned to the skull or, since the Icon, in a thermoplastic mask monitored by an infrared marker and a built-in cone-beam CT, which also allows treatment over several sessions. It treats brain metastases (routinely ten or more in one session), meningiomas, vestibular schwannomas, pituitary adenomas, arteriovenous malformations and trigeminal neuralgia, and it carries the longest follow-up of any radiosurgery platform.
Its strengths are sub-millimetre mechanical accuracy, the steepest dose fall-off outside the target and a workflow entirely dedicated to the brain. Its limits are that it treats only the head and upper neck, the sources decay and must be replaced every several years at large cost, the shielded room and radioactive inventory carry regulatory burdens, and linac-based radiosurgery on a well-commissioned C-arm or ZAP-X now achieves comparable clinical results without a cobalt source.
- Ablative dose, 1-5 fractions
- Converging beams
How it works
Many collimated cobalt-60 gamma beams converge on a stereotactically defined point so that dose sums at the target and falls off steeply outside it; frame or mask fixation with cone-beam CT holds the target at the focus.
- Sub-millimetre accuracy and steepest fall-off
- Many brain metastases in one session
- Decades of outcome data
- Intracranial targets only
- Cobalt sources decay and must be replaced
- Radioactive inventory, shielding and licensing
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