x-rays are most often used to treat some types of leukemia but sometimes gamma rays, there's also proton therapy (proton beam) I guess it would give atoms in cancer cells a shorter half life? but the point of the radiation is to knock off electrons from the atoms that make up the dna in cancer cells, to ionise them which destroys the dna (dna is the instructions the body uses to make a new cell) so the cell can't replicate