It is considered a mainstay of cancer treatment. It can be delivered in two ways. One method uses a beam outside the body, and is called external beam radiation. The other method is to implant radioactive material inside the body. This is known as brachytherapy.
Radiation affects molecules, proteins, and DNA in cells.
The cells become damaged and are then unable to function and divide normally. Cancer cells are more susceptible to radiation because they can't repair the damage. Other cancers are resistant to radiation. Before its toxicity was well known, radiation was used for many conditions much less serious than cancer. This happened mostly in the early part of the 20th century.