ABSTRACT
All stereotactic applications are characterized by the use of small radiation fields which may cause nonequilibrium conditions for electron transport. Small beams employed by various devices capable of delivering radiosurgery are created by specialized collimator assemblies in the form of precisely machined circular collimators or multileaf-collimator (MLC) systems. The older Gamma Knife models (U, B, and C) use 201 individual Co-60 sources, which are collimated by an internal (primary) collimator and a removable external semispherical assembly called a helmet, which has circular apertures of 4, 8, 14, and 18 mm. The collimating system focuses the individual beams of gamma radiation to a very precise focal point (the unit center point), and the superposition of all the beams at the focus delivers a tightly conformal, high-dose volume to the target with a rapid dose falloff away from the target edge. The new Gamma Knife Perfexion model has only one internal collimating system with 192 Co-60 sources distributed over eight movable sectors, and the final beam collimation is achieved with the appropriate alignment of sources over 4-, 8-, and 16-mm apertures to deliver focused radiation to the target.
