There are five current versions of the Leksell Gamma Knife (Models U, B, C, 4C and Perfexion®). Although the physical appearances of the models differ, the internal design leads to dose profiles that differ only slightly. Each system consists of six components: the radiation unit, the beam-focusing technology, the patient couch, an electric bed system, the control console, and the planning computer system.
In the C and 4C units, removable 4, 8, 14, or 18mm tungsten collimators with circular apertures are used to create various size focal volumes. Modification of the target radiation dose distribution can be achieved using multiple target points (isocenters) with different collimators, or use of beam plugging, or different beam angles. The newer Perfexion unit utilizes an internal collimation system with 4, 8 or 16mm isocenters.
Movement of the patient couch in and out of the radiation unit, and opening of the shielding door is performed with high precision. The patient’s head is moved in the focus point using very precise robotics.
Stereotactic computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), magnetoencephalography, or cerebral angiography are used for target determination, depending on the indication. The accuracy of dose delivery using the Gamma Knife as tested at UPMC was found to be approximately 0.25 millimeter. Mechanical accuracy as dictated by the manufacturer should be less than 0.3 millimeter.
A significant advantage of the Gamma Knife over other available radiosurgical systems is the ability to create conformal and irregular dose plans using multiple target points. Highly conformal radiosurgery is crucial to maintain low side effect rates by tailoring the effect to the irregular 3D geometry of the target volume; in addition the Gamma Knife has high selectivity, meaning that the radiation dose falls off outside of the target very steeply, thereby reducing risk of injury to adjacent structures.