Computer-guided dosimetry is specified to match the lesion. Lesions up to 3.5 centimeters in diameter may be treated. A cone that approximates the size of the lesion is placed in the collimator of the linear accelerator. Cones range in size from 12.5 mm to 40 mm. The head is secured to prevent movement while receiving treatment.
Radiation is given to the lesion from different directions called arcs.
A predetermined amount of radiation is delivered in an arc, then the treatment couch is rotated as is the collimator housing the cone and a predetermined amount of radiation is given. This sequence continues until the therapy is complete. The number of arcs used varies from 4 to 6 or more and takes approximately 30 minutes. If the patient has an AVM, an angiogram and CT are performed with the frame attached. The Gamma Knife consists of a sphere containing 201 Cobolt-60 sources.