Neurosurgery

Linear Accelerators

Linear accelerators, used for years in radiotherapy, have been modified by the addition of a secondary collimator system to perform radiosurgery. Single-focus multiple non-coplanar arc and conformal block techniques are the most widely used. These phrases need explanation to the neurosurgery reader. The former technique means that the radiation source is moved along a large arc, while pointing to the same center (the so-called "isocenter"), then a similar arc is drawn tilted a few degrees away from the first one. A series of arcs are being used to maximize the dose to the center (the target) and minimize the exposure of the surrounding tissues. The second technique involves manufacturing an irregularly shaped portal for the radiation, attempting to match the shape of the targeted lesion as if viewed from the direction of that beam. Similar blocks are made for each entry beam (usually five or six). A series of static, shaped beams are then used to irradiate the lesion from a number of angles. In order to improve conformality, in some centers multiple overlapping foci are used (in a similar fashion to that described for the gamma knife). However, the calculations and set-up of numerous fields are much more difficult with the moving source of a linear accelerator than when using a gamma knife, and this deters most centers from using this technique.

More flexibility is incorporated into the system if the linear accelerator is adapted with a micro-multileaf collimator. The device consists of a series of individually motorized tungsten leaves that can be positioned automatically to create any desired beam shape. This is effectively the same principle as the conformal block technique but avoids the need to make up specific blocks for each use. The relatively short collimation length and radiation transmission between the collimator leaves are factors that may degrade the sharpness of the final radiation dose gradient and could result in higher doses to normal adjacent brain tissues. The high cost and complexity of these additions resulted in a slow initial acceptance of this method. In the intensity-modulated radiotherapy (IMRT) technique, the dose is delivered in different intensities across the lesion. The collimator leaves dynamically open and close under computer control to selectively expose or shield portions of the tumor according to predetermined limits. Treatment planning for IMRT is complex. All components of the treatment and planning thus require computer control. This promising technique is in its early stages of development and is still under clinical evaluation.

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