Gamma knife radiosurgery: current status and review.

"Radiosurgery" is stereotactic neurosurgery in which the usual rigid needles or probes are replaced with beams of ionizing radiation directed toward an intracranial target so as to achieve local tissue destruction. No skin incision or skull opening is required. Lars Leksell originally defined the concept and over a prolonged research and development period created the "Gamma Knife." The Gamma Knife is a self-contained unit with 201 cobalt 60 sources arranged in a hemispheric array such that the emitted beams of radiation reach a common point of intersection. Interchangeable collimating devices allow variation of the diameter of the 50% isodose distribution from 5.0 to 24.2 mm. Larger radiation dose distributions can be achieved by multiple overlapping exposures. Initial radiation dose rate is between 300 and 400 cGy/min. Obliteration of arteriovenous malformations has been shown angiographically in 80% to 86% of optimally treated patients two years later. Growth arrest or actual decrease in tumor size has occurred in 86% of acoustic neuromas treated. Other lesions treated with the Gamma Knife have included pituitary adenomas, craniopharyngiomas, pineal region tumors, and ocular melanomas, as well as functional disorders.