ABSTRACT
Leksell's device, later popularly known as the Leksell Gamma Knife, was restricted to intracranial use only and required placement of the Leksell Model G stereotactic frame, at first with screws penetrating the skull and later with sharp pins. Metastatic brain tumors became a rising indication for Gamma Knife radiosurgery in the 1990s. The Gamma Knife Perfexion was introduced in 2006. The original Leksell Gamma Unit was invented to ablate small intracranial targets to treat chronic pain and movement disorder. The geometry of Gamma Knife unit makes it physically impossible to measure percent depth dose, and the concept would be meaningless due to the superposition of all 192 gamma ray beams. The radiation fluence of a Gamma Knife unit is usually characterized by the dose rate measured at the center of a 16-cm spherical phantom using an ionization chamber for the largest collimator. The off-axis ratio table for dose calculation in Gamma Knife radiosurgery is usually generated from Monte Carlo simulation.
