rus-32) and radiolabeled iodine (iodine-131) provided valuable information about the selectivity of proposed therapeutic regimens. Radioisotopes of iron and chromium were also valuable in applications in hematology. Red blood cell survival, iron physiology, and blood volume were some of the important contributions. In the early 1940s, phosphorus-32 and then sulfur-35 and iodine-131, were used to label antigens and antibodies. In the process of studying the behavior of iodine-131-labeled insulin, Berson and Yalow (1959) developed the sensitive assay system for blood components known as radioimmunoassay, the importance of which was recognized with a Nobel Prize in 1977.

This body of work laid the groundwork for modern biomedical and clinical research, in particular with reference to the tracer principle, which became an invaluable tool (Bizzell, 1966; Mirzadeh et al., 1992). The use of radioisotopes is unique in that it provides a method for measuring biochemical processes in vivo, especially in cases in which the process is easily saturated, since radiation makes it possible to detect and localize quantities as small as only a few thousand radiolabeled molecules. The development of generator technology in the 1960s marked another advance in nuclear medicine research and in clinical nuclear medicine. The Brookhaven National Laboratory (BNL) used fission-product molybdenum-99 to produce the first molybdenum-99/technetium-99m (99Mo/99mTc) generator, revolutionizing the field of nuclear medicine (Tucker et al., 1958). 99mTc, the isotope used in more than 80 percent of the diagnostic nuclear imaging studies performed today, is the short-lived "daughter" resulting from the decay of 99Mo. Simple devices now enable hospitals to extract 99mTc from the 99Mo/99mTc generators as needed, and instant kits provide prepackaged chemicals to simplify its incorporation into organic molecules.

Other reactor-produced radioisotopes continue to play a major role in research, and recent advances in many fields (such as molecular biology, including the Human Genome Project) could not have been accomplished without the use of 32P. In addition, many of the isotopes useful for therapeutic applications, such as strontium-89 for the palliation of metastatic bone pain, are produced in reactors. Two other reactor-produced radioisotopes, samarium-153 and rhenium-186, may also be of use in the treatment of bone cancer and are currently under clinical study. There is therefore a need to maintain a continuous supply of these isotopes both for the benefit of patients and to provide investigators with the tools needed to develop and improve such technologies.

CURRENT APPLICATIONS IN MEDICINE AND PHYSICAL AND LIFE SCIENCES

The peaceful use of radioisotopes has made great progress since 1911, when George de Hevesy demonstrated that various substances could be radiolabeled and subsequently "traced" by spiking his meals with some naturally occurring radioactivity to prove that his landlady was using leftovers instead of fresh food



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