Treatment of disease by radiation; also called irradiation. The radiation may be given in the form of a beam, or radioactive material (such as radium) may be contained in various devices that can be inserted directly into the tissues or into a body cavity. The radioactive material may also be given by injection into a vein or as a drink.

The most common condition for which radiation is used is cancer. Some cancers respond well to radiation therapy alone; others are treated best with radiation therapy in conjunction with surgery or anticancer drugs (or both). When a cancer is too far advanced for surgery, radiation therapy can often help to relieve symptoms.

In addition to killing cancer cells, radiation can also have a harmful effect on normal cells. But cancer cells are more susceptible because radiation acts best on cells that are dividing (multiplying) rapidly. The normal cells most commonly affected are those that divide rapidly—-such as those of the skin, the gastrointestinal tract, the reproductive organs, and the bone marrow.

Depending on where the beam of radiation is directed on the body, side effects from radiation can include diarrhea and vomiting, mouth ulcers, hair loss, and suppression of the bone marrow. Most of the side effects quickly disappear if the dose of radiation is reduced or if the treatment is stopped briefly. Modern techniques and equipment have reduced the frequency of side effects.