Definition:
external radiation therapy (...RAY-dee-AY-shun THAYR-uh-pee)
A type of radiation therapy that uses a machine to aim high-energy rays at the cancer from outside of the body. Also called external-beam radiation therapy.
Enlarge External-beam radiation therapy of the brain; drawing shows a patient lying on a table under a machine that is used to aim high-energy radiation. An inset shows a mesh mask that helps keep the patient's head from moving during treatment. The mask has pieces of white tape with small ink marks on it. The ink marks are used to line up the radiation machine in the same position before each treatment.

External-beam radiation therapy of the brain. A machine is used to aim high-energy radiation. The machine can rotate around the patient, delivering radiation from many different angles. A mesh mask helps keep the patient’s head from moving during treatment. Small ink marks are put on the mask. The ink marks are used to line up the radiation machine in the same position before each treatment.

Enlarge External-beam radiation therapy of the head and neck; drawing shows a patient lying on a table under a machine that is used to aim high-energy radiation at the cancer. An inset shows a mesh mask that helps keep the patient's head and neck from moving during treatment. The mask has pieces of white tape with small ink marks on it. The ink marks are used to line up the radiation machine in the same position before each treatment.

External-beam radiation therapy of the head and neck. A machine is used to aim high-energy radiation at the cancer. The machine can rotate around the patient, delivering radiation from many different angles to provide highly conformal treatment. A mesh mask helps keep the patient’s head and neck from moving during treatment. Small ink marks are put on the mask. The ink marks are used to line up the radiation machine in the same position before each treatment.