About 60% of all cancer patients are treated with some form of external radiation therapy. Also called radiotherapy, this treatment delivers radiation—X-rays or gamma rays—directly to the cancer site.

Radiation doses are based on the type, stage, and location of the tumor as well as on the patient’s size, condition, and overall treatment goals. Doses are given in increments, usually 3 to 5 times a week, until the total dose is reached.

The goals of radiation therapy include cure, in which the cancer is completely destroyed and not expected to recur; control, in which the cancer doesn’t progress or regress but is expected to progress at some later time; or palliation, in which radiation is given to relieve signs and symptoms (such as bone pain, seizures, bleeding, and headache) caused by the cancer.

External beam radiation therapy is delivered by machines that aim a concentrated beam of high-energy particles (photons and gamma rays) at the target site. Two types of machines are commonly used: units containing cobalt or cesium as radioactive sources for gamma rays, and linear accelerators that use electricity to produce X-rays. Linear accelerators produce high energy with great penetrating ability.

Radiation therapy may be augmented by chemotherapy, radiation implant therapy (brachytherapy), or surgery, as needed.