Radiation used for cancer treatment is called ionizing radiation because it forms ions (electrically charged particles) in the cells of the tissues it passes through. 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 form ions.
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.
Sources of radiation
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). Photon beams of energy affect the cells along their path as they pass through the body to get to the cancer. This is by far the most common type of radiation treatment in use today.
- Electron beams or particle beams are also 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 charged 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. 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 and for inoperable tumors. They can sometimes be helpful when other forms of radiation therapy don’t work. Very few facilities in the United States offer this type of treatment. Its use has declined over the years partly because of problems with getting the beams on target. Because neutrons can cause more DNA damage than photons, the effects on normal tissue may be more severe. Beams must be aimed carefully and normal tissue protected. Neutron beams are showing great promise in research with salivary gland cancers that can’t be cured with surgery.
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