I do know a bit about radiation therapy. Unlike chemo it actually targets cancer cells. It works like this.

1. Patient is injected with a low dose radioactive compound. Cancer cells EAT EVERYTHING (including sugar). They absorb these guys like candy.

2. The radioactive compound inside the cancer cells is a bomb waiting for a trigger. A photon (X-ray) or proton/neutron beam is directed at the tumor sites.

3. The beam causes the radioactive compound to split up (a nuclear fission reaction) inside the cancer cells. The cancer cells are destroyed by the kinetic energy of the fission products.

4. Normal cells are spared because A) they do not take in as much as the radioactive compound (no fission occurs because no compound). And B) that the beam is directed at tumor sites.