Radiation therapy, also called radiotherapy, attacks cancer cells with X-rays or other high energy particles from radioactive substances. Radiotherapy is often used to treat cancers of the bladder, prostate, head, and neck. Radiation kills cancer cells, but it also kills normal cells. One type of radiotherapy, called external beam radiotherapy, sends X rays into the target tissue to destroy cancer cells. Machines called linear accelerators produce the X rays at increasingly greater energy. The higher the energy of the X rays, the deeper the beam can penetrate into the body to reach cancer tumors.

Physicians use CT scans and MRI to determine the exact size and shape of a cancerous tumor. In a technique called three-dimensional conformal radiation therapy, the radiation beam is shaped to exactly match the cancerous tumor. Physicians can then deliver a high dose of radiation to the tumor with little radiation exposure to healthy tissue.

A technique called stereotactic radiosurgery is often used to treat cancer tumors located in the brain that cannot be removed through conventional surgery. This technique uses a concentrated dose of gamma rays-a form of high energy radiation-from a radioactive source. With stereotactic radiosurgery, a total of 201 radiation beams intersect on a target area of cancer cells within the body. This precise technique destroys the cancer cells while sparing adjacent healthy tissue.

Brachytherapy is often used to treat cancers of the prostate and brain. In this type of radiotherapy, radiation comes from small capsules, called seeds, of radioactive material implanted by a surgeon close to, or within, the cancer tumor. The radiation only penetrates a short distance, so nearby healthy tissue is not harmed while the cancerous cells are killed.