Radiation Therapy Principles

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Types of radiation used to treat cancer

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. It creates ions by removing electrons from atoms and molecules. This can kill cells or change genes so the cells stop growing.

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 in cancer treatment
  • Particle radiation (such as electrons, protons, neutrons, carbon ions, 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 and location.

Photon radiation

By far the most common form of radiation used for cancer treatment is a high-energy photon beam. This comes 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 go through the body to get to the cancer, pass through the cancer, and exit the body.

Particle radiation

Electron beams or particle beams are also produced by a linear accelerator. Electrons are negatively charged parts of atoms. They have a low energy level and don’t penetrate deeply into the body, so this type of radiation is used most often to treat the skin, tumors, and lymph nodes that are close to the surface of the body.

Proton beams are a form of particle beam radiation. Protons are positively charged parts of atoms. They cause less damage to tissues they pass through but are very good at killing cells at the end of their path. Because of this, proton beams are thought to 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 not widely available.

Neutron beams are used for some cancers of the head, neck, and prostate and for certain inoperable tumors. A neutron is a particle in many atoms that has no charge. Neutron beam radiation can sometimes help when other forms of radiation therapy don’t work. Few facilities in the United States offer it, and use has declined partly because of problems getting the beams on target. Because neutrons can damage DNA more than photons, effects on normal tissue can be more severe. Beams must be aimed carefully and normal tissue protected. Still, neutron beams show great promise with salivary gland cancers that can’t be cured with surgery.

Carbon ion radiation is also called heavy ion radiation because it uses a particle that is heavier than a proton or neutron. The particle is part of the carbon atom, which itself contains protons, neutrons, and electrons. Because it’s so heavy, it can do more damage to the target cell than other types of radiation. As with protons, the beam of carbon ions can be adjusted to do the most damage to the cancer cells at the end of its path. But the effects on nearby normal tissue can be more severe. This type of radiation is only available in a few centers in the world. It can be helpful in treating cancers that don’t usually respond well to radiation (called radioresistant).

Alpha and beta particles are mainly produced by special radioactive substances that may be injected, swallowed, or put into the body of a person with cancer. They’re most often used in imaging tests, but can be helpful in treating cancer. You can read more about these in the section called “Radiopharmaceuticals”.


Last Medical Review: 10/23/2013
Last Revised: 10/23/2013