Gamma Knife radiosurgery

Non-invasive write · writes

Gamma Knife radiosurgery is a device holding sealed cobalt-60 sources (192 in one version, 201 in another) whose gamma rays pass through circular tungsten openings, with nominal collimator sizes of 4, 8 or 16 mm, directing the radiation to a single focus. By combining several focal points and collimator sizes, it delivers a radiation dose shaped to a three-dimensional target in the head. Target coordinates come from markers on a head frame or from built-in cone-beam CT imaging, while a frame or mask holds the head still. For a 4 mm Perfexion collimator, the single-shot dose-profile FWHM is 5.04–6.16 mm across axes in the planning system and an ellipsoid model.

Spatial resolution

5.04–6.16 mm (focal extent)

Temporal resolution

n/a

Field of view

44–19900 mm³ (treated volume)

Reads

n/a

Writes

Gamma radiation (cobalt-60)

Destructive?

destructive

Prerequisites

other: Immobilize the head using either an invasive fixed G-frame applied under local anaesthesia or the Icon frameless mask workflow.; anesthesia: Use local anaesthesia when applying the invasive stereotactic G-frame.; other: Acquire stereotactic imaging to define coordinates: MRI or CT with a fiducial indicator box, or Icon on-board cone-beam CT.; contrast agent: Administer gadolinium contrast when a contrast-enhanced planning MRI is used.

Theoretical temporal resolution

n/a

Theoretical field of view

1.23 L (reachable envelope)

Description

Gamma Knife radiosurgery is stereotactic treatment of a small intracranial target with many thin beams of gamma radiation from sealed cobalt-60 sources, all converging on one focus. The beams converge on the target, which keeps the damage to the surrounding tissue to a minimum. In the Icon unit, 192 sources are arranged in eight motorized sectors; to treat, the sectors move over a circular tungsten collimator with nominal collimator sizes of 4, 8 or 16 mm, so that every beam points at a single radiative focus. The older Model B has 201 sources. A treatment plan usually combines one or more isocenters (focus positions) using different collimator sizes to conform the dose to the three-dimensional volume of the target; the number of isocenters depends on the size, shape and location of the target. For a trigeminal nerve target, the proposed rationale for pain relief is focal axonal degeneration of the nerve, destruction of ion channels, and electrophysiological blocking soon after irradiation. Target coordinates come from a stereotactic image set: coordinates derived from a fiducial indicator box on a stereotactic head frame (the G-frame) in framed workflows, or on-board cone-beam CT in frameless workflows, which use a mask made for the patient. The Icon unit adds cone-beam CT and intra-fraction motion management to the Perfexion design. The mechanical treatment range of the Perfexion unit is 160, 180 and 220 mm along X, Y and Z, against 100, 120 and 165 mm for earlier models. For the 4 mm Perfexion collimator, the single-shot dose-profile FWHM is 5.04–6.16 mm across axes in the planning system and an ellipsoid model.

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