The Gamma Knife
The gamma knife uses an array of 201 Co60 sources, which are evenly distributed around a hemispherical source core. Each source (together with its associated collimator housing) produces a narrow beam of gamma radiation, and these are all directed towards a common focal spot at the center of the hemisphere ("isocenter"). The cross-firing of 201 radiation beams results in a sharp focus in which the central radiation intensity is high, and the intensity of dose falls rapidly with distance in any direction.
The size of the focal radiation field is controlled by the addition of secondary collimators, which are housed in a "helmet" situated at the head of the patient couch. Four collimator sizes are available, producing fields with nominal sizes of 4 mm, 8 mm, 14 mm and 18 mm.
A simple, single-field treatment requires the target tissue (e.g. entry zone of the trigeminal nerve for trigeminal neuralgia) to be accurately positioned at the center of the helmet collima-tor system where the fine radiation beams converge. This can be achieved with 0.1mm precision. The couch, helmet system and patient are then moved into the central body of the gamma knife. The treatment begins when the secondary collimators align with the sources.
Typically, the treatment of an intracranial lesion will require the overlapping of a number of foci or fields of radiation. After each field of radiation, the patient couch withdraws from the treatment position, and the position of the patient is adjusted to target an adjacent portion of the tumor. The latest C-model gamma knife offers an automatic positioning system for the head, both reducing human error and increasing precision. The ease with which this latest model affords treatment to many target points in succession encourages the planner to use a larger number of isocenters. This allows more complex plans using more isocenters to be formulated, the increased number of isocenters allowing greater conformity in matching the treatment volume to the tumor whilst sparing the surrounding tissues.
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