When scientists speak about particle accelerators it's usually in reference to quantum physics, space applications or other experimental functions. What most people don't realize is that over half of the world's 35,000 particle accelerators are used to treat cancer. Not for space, or new forms of energy, but for treating several forms of cancer. Beginning with the discovery of X rays, which damage cancer through ionization of tumors, experts have realized that particle physics can play a role in medical treatment.
The problem with the photons in X-rays damaging the DNA in tumors is that they injure everything they are aimed at, even healthy tissue and they pass through the body leaving a wake of damage. For better precision and less collateral damage, protons are a more efficient particle to use. Particle accelerators can take protons, focus them into high energy rays and then aim them at the tumor. A law of physics known as the Bragg Peak can be manipulated so that the protons deposit specific amounts as the enter the body, and keep depositing them, at a lower amount as they travel through to the targeted tumor. At this point, they essentially "run out of gas" and stop, having done their duty without negatively impacting health tissue.