A high-energy photon beam is by far the most common form of radiation used for cancer treatment. It is the same type of radiation that is used in x-ray machines, and comes from a radioactive source such as cobalt, cesium, or a machine called a linear accelerator (linac, for short).
The speed of radioactive particles is also an important factor in medical use. Beta particles travel very fast. This, combined with their small size, gives them significant penetrating power. In cancer treatment, for example, beams of beta particles can be created outside the patient's body and directed at tumors.
