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A single x-ray or similar type beam can pass through a human body with little or no damage. Has anyone ever tried to use a second beam that is phased to have no harmful reactions with the tissue except at the exact spot where two or more beams collide. I know they use sound waves to smash gall stones but I have never heard of using two waves that interact on each other. Is it even possible to do this? They also use chemicals collected in a tumour that will react when hit by a beam.

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"A single x-ray or similar type beam can pass through a human body with little or no damage" This is a bit of a simplification in and of itself. There is damage, but it is modest and dispersed. –  dmckee Aug 18 '12 at 5:35

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X-rays don't really interfere like that, at least not in your body and it wouldn't really help anyway.

You could focus a wide cone shaped beam so that it only had sufficent power at a small area and did less damage to the tissue in front. I don't know if this is done for treatment but we do use it for CT scans to get higher resolution in a small region

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Gamma knife uses a multitude of beams overlapping at the focus to ablate tumors. But yes there is no interference involved, it's just to avoid concentrating the energy outside of the target. –  user2963 Aug 18 '12 at 3:34
@zephyr Yep. And these days heavy particle therapies (protons, alphas, even carbon nuclei) are all the rage because they do the job even better, dumping their energy almost entirely in the tumor. –  dmckee Aug 18 '12 at 6:25

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