Radiation is a form of intense energy generated by a radioactive substance, such as cobalt, or by specialized equipment, such as an atomic particle (linear) accelerator.
Radiation preferentially kills cells that divide rapidly and cells that have difficulty repairing their DNA (nuclear material). Cancer cells divide more often than normal cells and often cannot repair damage done to them by radiation. Therefore, cancer cells are more likely than most normal cells to be killed by radiation. Nonetheless, cancer cells differ in how easily they are killed by radiation. Some cells are very resistant and cannot be effectively treated with radiation.