Energy’s (DOE’s) Office of Science, and by two Canadian facilities, TRIUMF and Chalk River.
Worldwide, the molybdenum-99/technetium-99m radionuclide pair is used in four out of five, or in about 12 million diagnostic-imaging procedures in nuclear medicine every year. However, the reactors that have been producing molybde-num-99 are approaching the end of their useful lives, which is expected to trigger an “isotope crisis.” One of the reactors, the Canadian National Research Universal (NRU) reactor at Chalk River, is scheduled to stop isotope production in 2016, while potential replacement reactors around the world may not be available until 2020. Research is now focused on exploring accelerator-based production of molybdenum-99 as an alternative technology using, among other reactions, the 100Mo(g,n)99Mo and the 100Mo(p,2n)99mTc reactions.
Another option centers on rhenium-186, which has a favorable half-life (t1/2 = 90 hours) and emits beta decay electrons of 0.9 MeV with a 10 percent branch emitting a gamma-ray with energy similar to that of technetium-99m. Since rhenium is in the same chemical family as technetium, much of the technology developed for technetium-99m can be applied to rhenium-186. Current efforts are concentrated on reactor production of rhenium-186 via the 185Re(n,g) reaction, followed by mass separation to yield a sample with the high specific activity needed for therapy (see Box 3.1).
Radiopharmaceuticals have been developed that can be targeted directly at the organ being treated. These therapy radiopharmaceuticals rely on the destructive power of ionizing radiation at short ranges, which minimizes damage to neighboring organs.
A frontier direction is targeted radiopharmaceuticals. This involves attaching a relatively short-lived radioactive isotope that decays via high-energy transfer radiation (alpha-particle emission, for example) to a biologically active molecule, like a monoclonal antibody that has a high affinity for binding to receptors on cancer tumors. When the radioactive nuclei decay, the radiation they produce loses energy quickly and because it does not travel far, a lethal dose of radiation is delivered only to adjoining tumor cells. By careful construction of the targeting molecule, the radioactive nuclei will pass through the body quickly if they do not bind to tumor cells, thus minimizing the exposure of healthy tissue to the high-energy transfer radiation. Presently, the most common radionuclides are iodine-131 and yttrium-90, though neither is ideal. Two radiopharmaceuticals, Bexxar (using iodine-131) and Zevalin (using indium-111 or yttrium-90), are now in use to treat non-Hodgkins lymphoma.
Many research efforts are focused on the production of alternative isotopes