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 visible 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:

  • Photon radiation (x-rays and gamma rays)
  • 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 more energy, the more deeply the radiation can penetrate (get into) the tissues.

The way each type of radiation behaves is important in planning radiation treatments. The radiation oncologist (a doctor specially trained to treat cancer with radiation) selects the type of radiation that’s most suitable for each patient’s cancer type and location.

Photon radiation

A high-energy photon beam is by far the most common form of radiation used for cancer treatment. It is the same type of radiation that is used in x-ray machines, and comes from a radioactive source such as cobalt, cesium, or a machine called a linear accelerator (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 then 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, as well as 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 release their energy only after traveling a certain distance and cause little damage to the tissues they pass through. This makes them very good at killing cells at the end of their path. So, proton beams are thought to be able to deliver more radiation to the cancer while doing less damage to nearby normal tissues.

Proton beam radiation therapy is used routinely for certain types of cancer, but still need more study in treating others. It requires highly specialized equipment and is not widely available.

Some of the techniques used in proton treatment can also expose the patient to neutrons (see below).

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 it can be difficult to target the beams effectively. Because neutrons can damage DNA more than photons, effects on normal tissue can be more severe.

Carbon ion radiation can be helpful in treating cancers that don’t usually respond well to radiation (called radioresistant). It’s also called heavy ion radiation because it uses a particle that’s 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.

Alpha and beta particles are mainly produced by special radioactive substances that may be injected, swallowed, or put into the body. 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: 10/27/2014
Last Revised: 10/27/2014