The use of ionizing radiation in the treatment of cancer and some diseases of bone marrow and blood. X-rays and gamma radiation used in diagnostic radiology have low energy so that they do not damage the tissues through which they pass. Radiotherapy, however, makes use of the property of ionizing radiation that diagnostic radiology tries to avoid; that is, it has to have sufficient energy to kill the cells on which it falls.

Ionizing radiation kills cells indiscriminately. Therefore when a cancerous tumour is irradiated, the radiation will kill a number of normal tissue cells in the vicinity of the tumour. Since high doses are usually required to kill tumours, steps must be taken to limit the damage to normal tissue. This is achieved by a method called fractionation, in which the necessary radiation dose is divided into a number of small fractions administered over a period. This allows the normal tissue partially to recover between treatments. Since, in some cases, cancer cells are more vulnerable to radiation than normal cells, normal tissue recovery is more rapid, enabling fractionation to increase the ability of the patient to tolerate the therapy.

A radiation dose may be administered to the patient either from outside the body or inside the body. Teletherapy is the use of radiation beams external to the body, and brachytherapy is the use of small-volume sealed radionuclide sources implanted near the tumour.