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A linear accelerator, or “linac,” generates X-ray radiation via the acceleration of electrons that are extracted off the surface of a heated metal disk. The electrons are accelerated through a vacuum chamber by microwaves to nearly the speed of light, an action that greatly boosts their energy levels. These speeding electrons bombard a metal target, usually tungsten, causing it to emit X rays, which are collimated into pencil-thin beams that can be adjusted to cover the 4- to 25-million-volt spread of energies needed to penetrate tumors. The beams are intense, meaning they contain a large number of X-ray photons. Varian’s Clinac EX linac can deliver a dose rate of X rays up to 600 centigray per minute and concentrate them on an area 2 millimeters in diameter, which is about the size of this spot.