Radiation Therapy is a treatment for cancer that typically involves the use of "high-energy" x-rays, ("photons"), although other types of radiation particles such as electrons and protons can also be used.  Whereas the energy of radiation used in diagnostic radiology (such as those use to see your bones) are in the kilo-volt range, radiation used in the treatment of cancer is in the mega-volt range, capable of destroying malignant tissues. (ANSWERED by Dr. Shawn Zimberg of Advanced Radiation Centers)