steps. the combination of the movements of the gantry and of the couch allow the computerized planning of the volume of tissue that is going to be irradiated. devices with a high energy of 6 mev are commonly used for the treatment of the brain, due to the depth of the target. the diameter of the energy beam leaving the emission head can be adjusted to the size of the lesion by means of collimators. they may be interchangeable orifices with different diameters, typically varying from 5 to 40 mm in 5 mm steps, or multileaf collimators, which consist of a number of metal leaflets that can be moved dynamically during treatment in order to shape the radiation beam to conform to the mass to be ablated. as of 2017 linacs were capable of achieving extremely narrow beam geometries, such as 0.15 to 0.3 mm. therefore, they can be used for several kinds of surgeries which hitherto had been carried out by open or endoscopic surgery, such as for trigeminal neuralgia. long-term follow-up data has shown it to be as effective as radiofrequency ablation, but inferior to surgery in preventing the recurrence of pain. the first such systems were developed by john r. adler, a stanford university professor of neurosurgery and radiation oncology, and russell and peter schonberg at schonberg research, and commercialized under the brand name cyberknife. proton beam therapy main article: proton therapy protons may also be used in radiosurgery in a procedure called proton beam therapy (pbt) or proton therapy. protons are extracted from proton donor materials by a medical synchrotron or cyclotron, and accelerated in successive transits through a circular, evacuated conduit or cavity, using powerful magnets to shape their path, until they reach the energy required to just traverse a human body, usually about 200 mev. they are then released toward the region to be treated in the patient's body, the irradiation target. in some machines, which deliver protons of only a specific energy, a custom mask made of plastic is interposed between the beam source and the patient to adjust the beam energy to provide the appropriate degree of penetration. the phenomenon of the bragg peak of ejected protons gives proton therapy advantages over other forms of radiation, since most of the proton's energy is deposited within a limited distance, so tissue beyond this range (and to some extent also tissue inside this range) is spared from the effects of radiation. this property of protons, which has been called the "depth charge effect" by analogy to the explosive weapons used in anti-submarine warfare, allows for conformal dose distributions to be created around even very irregularly shaped targets, and for higher doses to targets surrounded or backstopped by radiation-sensitive structures such as the optic chiasm or brainstem. the development of "intensity modulated" techniques allowed similar conformities to be attained using linear accelerator radiosurgery. as of 2013 there was no evidence that proton beam therapy is better than any other types of treatment in most cases, except for a "handful of rare pediatric cancers". critics, responding to the increasing number of very expensive pbt installations, spoke of a "medical arms race" and "crazy medicine and unsustainable public policy".