[ RadSafe ] RE: medical issues

Stabin, Michael michael.g.stabin at Vanderbilt.Edu
Tue Jan 27 14:55:48 CST 2009

> The biological half-lives of radionuclides like Cs-137, Tl-201, Tc-99m and all others can be easily found within a minute at Google

Well, not really, if you account for the many different compounds that the nuclides may be labeled to. The elemental forms, yes, but Tc-99m is labeled to dozens or hundreds of compounds, each of which has its own unique metabolism. This has been well characterized by the ICRP Task Group on Radiopharmaceutical Dosimetry, and we at RADAR try to keep their recommendations updated on our website (www.doseinfo-radar.com, see the nuclear medicine pages).

> What I am rather surprised is, that so much "radioactivity" should be attributed to impurities in the radionuclides used. To my knowledge there
are usually limits set for the concentration of long-lived radionuclides in the production - of course on radiation protection grounds.

In nuclear medicine, the levels of activity are higher than many are used to dealing with in other areas of health physics, as the half-lives are shorter (remember, Cs-137 is not a nuclear medicine nuclide), and the radiation dose per unit activity is much lower. High levels of activity are used to produce as high a quality of image can be obtained (typically hundreds of MBq), or in therapy to deliver high doses to malignant tissues (high numbers of GBq). In medicine, there are no legislative limits on radiation dose for patients. Radiopharmaceuticals are approved (in the USA) by the Food and Drug Administration, and physicians can vary the amount of activity, and thus radiation dose, given to any patient within reasonable limits to ensure that good medical information is obtained to diagnose or treat disease. Impurities in radiopharmaceutical products are tolerated to certain levels, and watched closely. For years we had a lot of I-124 in I-123 products, which is problematic dosimetrically. The I-123 production methods were changed, and this situation was significantly improved. The contaminants in Tl-201 products are quite detectable, but don't contribute all that much to radiation dose, see for example J Nucl Med. 46(3):502-508, 2005. Contaminants in In-111 products were once quite a dosimetry issue for labeled blood cells (J Nucl Med 26:1090 1093, 1985), but now Tc-99m labeled products have taken over much of these imaging studies.

By the way, I would like to endorse again the excellent analysis by Pat Zanzonico of the Memorial Sloan Kettering Cancer Institute on the possible numerical **benefits** of medical examinations involving radiation, see http://hps.org/hpspublications/articles/Benefitsofmedradexposures.html. My name only appears on this because I helped organize the web page, the brilliant analysis was all written by Pat.

Michael G. Stabin, PhD, CHP
Associate Professor of Radiology and Radiological Sciences
Department of Radiology and Radiological Sciences
Vanderbilt University
1161 21st Avenue South
Nashville, TN 37232-2675
Phone (615) 343-4628
Fax   (615) 322-3764
e-mail     michael.g.stabin at vanderbilt.edu
internet   www.doseinfo-radar.com
"I am realistic -- I expect miracles." - Wayne Dyer

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