Answer to Question #2886 Submitted to "Ask the Experts"

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The term "external beam radiation" generally refers to the use of treatment machines, specifically linear accelerators, to treat disease. The source of radiation is external to the body, typically at a distance of 1 meter. The source of radiation is a high atomic number material such as tungsten, which is used as a target in the head of the treatment machine. The target is subjected to a stream of electrons, and high-energy x rays are produced. Electrons may also be used by themselves without the production of x rays by removing the target from the stream of electrons. For modern linear accelerators, one or more photon or x-ray energies may be available, e.g., 6 MV and 15 or 18 MV, and one or more electron energies may also be available. At photon energies above 10 MV, photoneutrons may be produced in the high atomic number materials. These neutrons are a contaminant in the treatment beam and are not being used for treatment. There are some research accelerators using protons or other forms of radiation for external beam treatment, but these are not yet in common use.

"IMRT" or Intensity Modulated Radiation Therapy, is a technique which attempts to tailor the treatment field to the shape and contour of the tumor volume. Tumors are never rectangular or square as are typical treatment fields. IMRT makes use of special collimators, called multileaf collimators, which are able to mimic the shape and size of the tumor. By using these techniques, the dose to other tissues is reduced and higher doses can be delivered to the tumor itself. There are two recently published books on IMRT, A Practical Guide to Intensity-Modulated Radiation Therapy and Intensity-Modulated Radiation Therapy, the State of the Art, both available from Medical Physics Publishing.

Brachytherapy is a treatment technique that uses sealed sources of radioactive materials placed in tissue or body spaces to deliver radiation dose at short treatment distances. Modern brachytherapy uses afterloading techniques in which sources are loaded into a device such as a catheter or applicator previously placed in tissue or in a body space after a treatment plan has been developed and optimized by discussion between the radiation oncology physicist and the radiation oncologist, the treating physician. You may wish to visit the website of the American Brachytherapy Society for more information. Most of the sources used for brachytherapy are made in reactors, and most of these reactors are outside the United States.