Stereotactic Forms Of Teletherapy

Stereotaxis is the technique used to localize a target within surrounding normal tissue using a localization device equipped with a fixed fiducial system. These treatment technologies vary in the immobilization device, fiducial system, and dose delivery, but share a common theme of highly conformal therapy delivered to an immobilized target localized with sophisticated imaging. Stereotactic localization is frequently used with single fraction (stereotactic radiosurgery) or fractionated external beam radiation therapy (stereotactic radiotherapy). "Gamma knife" units have numerous radioactive cobalt sources in a hemispherical helmet, aligned so that the sources are all directed at the center of the sphere (35). Sources can be blocked or collimated to produce the desired dose distribution. Alternatively, linear accelerators can be modified to deliver stereotactic treatments, typically with the use of mounted, small multileaf collimators and gantry arc rotations around the isocenter (36).

For most intracranial stereotactic treatments, a rigid headframe is fixed to the patient's skull. The headframe typically includes an attachable fiducial device, which is used to define a coordinate system for target location in reference to the head-frame. This fiducial system allows the translation of 2D CT or magnetic resonance imaging (MRI) data into a 3D volume that can be localized in reference to the head-frame, and treatments planned in reference to the headframe. During treatment delivery, the headframe is fixed to the treatment table, ensuring the accuracy of alignment and preventing interfraction movement. This system allows for highly accurate delivery, in the order of 1 to 2 mm (36).

Various other forms of stereotactic therapies are in development, including CyberknifeTM, which is a frameless robotic linear accelerator delivery system designed to deliver arcs (37,38). A single treatment with this device can include hundreds or thousands of beams delivered with multiple isocenters, allowing a high degree of conformality with rapid fall-off of dose beyond the target tissue. The system mandates accurate target localization, typically accomplished with implanted fiducial markers.

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