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.
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