Gamma Knife surgery uses multiple tiny beams of radiation to shrink tumors. The left brain scan shows a pretreatment image of a noncancerous tumor — an image enhanced by the use of a special medium, called a contrast agent. At six months after treatment (middle image), the tumor appears slightly larger but doesn't take up as much of the contrast medium — it isn't as bright in the center. This indicates a positive treatment effect. At seven years (right scan), the tumor appears much smaller.
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