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07 October 2008

Answer to Question #54 Submitted to "Ask the Experts"

Category: Radiation Basics — Radiation Quantities and Units

The following question was answered by an expert in the appropriate field:

I was reading an article that used units that I was unfamilar with. Can you tell me what "mgRaeq" stands for?

The term "mg Ra eq" stands for "milligrams radium equivalent." Since the unit "curie" was defined in terms of the best estimate of the activity of a gram of radium, radium sources are often described in terms of milligrams rather than millicuries. The original brachytherapy work was done with radium (radium-226 plus daughters) and thus dose tables, treatment plans, protocols, etc., were in terms of radium. With the replacement of radium by other sources such as cesium-137, these sources have been described in terms of "radium equivalent" in order to facilitate use of the existing "institutional knowledge." Harold Johns and John Cunningham, in the Fourth Edition of The Physics of Radiology (Charles C. Thomas Publisher, 1983), in the section "Dose Prescription in Brachytherapy Treatments" on pages 462-464, discusses some of the qualifying conditions, gives a cesium-137 example, and presents a table containing radium equivalences for five radionuclide sources. There is probably more information available in other, more recent references, but this is the one I had readily available on my shelf.

Genevieve S. Roessler, PhD
Editor, Health Physics Society Web Site

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