They use radiation because it's a very directed way to cause tissue damage. When it comes to cancer, you have to realize that cancer cells are generally harder to kill than healthy cells. Most Chemotherapy exploits cancer's high metabolism with "poison food" to have the cancer build up deadly levels of toxins faster than the regular cells, but that becomes a careful game of balancing total toxins in the body vs the concentrated stuff.
With Radiotherapy they use some other more direct methods. The proton method is like how others described. Other radio therapy usees multiple beams from several angles to "overlap" each other meaning one area gets 2 or more beams of radiation saturating it with high levels while surrounding tissue takes less.
As with most medical systems, a good portion of the danger is the human operating it: does the tech know how to math, and did they RTFM? Sometimes, they don't.
All in all it's a game of trying to deathray a very specific part of a human while leaving the other parts alone.
Edited by GAHD, 11 December 2018 - 12:56 PM.