Radiosurgery that can destroy an intracranial target by directing gamma radiation at the lesion, while attempting to spare adjacent healthy tissue. The gamma knife consists of 201 cylindrical gamma ray (cobalt 60) beams designed to intersect at the target lesion, resulting in about 200 times the dose of any single beam aimed at the periphery. The area to be treated is carefully identified with neuroimaging before the gamma knife is used and the proper dose of gamma energy calculated. The procedure takes about 2 to 3 hr, with the patient under mild sedation, given intravenously, and local anesthesia. The gamma knife can be used to treat primary and metastatic brain tumors, trigeminal neuralgia, arteriovenous malformations, and other lesions. Complications include seizures, confusion, paralysis, nausea and vomiting, other radiation reactions, and radiation necrosis of normal brain tissue, but the incidence of side effects is no greater than with other brain irradiation or neurosurgical techniques.