Radiation used for cancer treatment is called ionizing radiation because it forms ions in the cells of the tissues it passes through as it dislodges electrons from atoms. (Ions are atoms that have taken on an electric charge by gaining or losing an electron.) This can kill cells or change genes so the cells cannot grow.

Other forms of radiation such as radio waves, microwaves, and light waves are called non-ionizing. They don't have as much energy and are not able to ionize cells.

Ionizing radiation can be sorted into 2 major types:

• photons (x-rays and gamma rays), which are most widely used
• particle radiation (electrons, protons, neutrons, alpha particles, and beta particles)

Some types of ionizing radiation have more energy than others. The higher the energy, the more deeply the radiation can penetrate (get into) the tissues. The way a certain type of radiation behaves is important in planning radiation treatments. The radiation oncologist (a doctor specially trained to treat cancer patients with radiation) selects the type and energy of radiation that is most suitable for each patient's cancer.

The more common sources of radiation used for cancer treatment are:

• High-energy photons that come from radioactive sources such as cobalt, cesium, or a machine called a linear accelerator (or linac, for short.) This is by far the most common type of radiation treatment in use today.

• Electron beams or particle beams that are produced by a linear accelerator. These are used for tumors close to a body surface since they do not go deeply into tissues.

• Proton beams are a newer form of particle beam radiation. Protons are parts of atoms that cause little damage to tissues they pass through but are very good at killing cells at the end of their path. This means that proton beams may be able to deliver more radiation to the cancer while causing fewer side effects to normal tissues nearby. Protons are used routinely for certain types of cancer, but still need more study in treating others. Some of the techniques used in proton treatment can also expose the patient to neutrons (see below). Proton beam radiation therapy requires highly specialized equipment and is currently only offered in certain medical centers. See the section, "What's new in radiation therapy?" for more about protons.

• Neutron beams are used for some cancers of the head, neck, and prostate. They can sometimes be helpful when other forms of radiation therapy don't work. Their use has declined over the years because of the rather severe long-term side effects they cause.

Go back to Radiation Principles.

Last Medical Review: 07/17/2009
Last Revised: 07/17/2009