In recent years, hospitals worldwide have been grappling with short supplies of technetium-99 (Tc-99), the most commonly used radioisotope in medical imaging scans. But help may be at hand: A team of Canadian researchers reported today that they've made critical progress in developing a stable new supply of the isotope.
Tc-99 is currently produced in nuclear reactors fueled with highly enriched uranium, which has raised concerns that the nuclear fuel could be intercepted by terrorists to make a nuclear weapon. The new setup produces Tc-99 with a medical cyclotron, thereby eliminating proliferation concerns. But economic and technical considerations may make it more practical for shoring up Tc-99 supplies in Canada than in the United States.
Two nuclear reactors, one in Canada, the other in The Netherlands, currently produce 60% of the world's supply of molybdenum-99 (Mo-99), an unstable radioactive isotope that decays into Tc-99. Once produced, Mo-99 is dissolved in a liquid and shipped to hospitals worldwide. Over 2 weeks, the Mo-99 decays into Tc-99, which is separated out and injected into patients for cardiac scans and other routine imaging procedures. Doctors worldwide carry out more than 30 million such procedures a year. Tc-99 cannot be stockpiled, because it has only a 6-hour half-life.
In 2009, the Canadian and Dutch reactors were forced to shut down temporarily, crimping Tc-99 production. Supplies are now back to normal, but both reactors are slated to close in the next several years, and governments around the world are scrambling to come up with new Tc-99 sources.
At the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW) in Vancouver, Canada, Paul Schaffer, head of nuclear medicine at TRIUMF, a nuclear and particle physics laboratory in Vancouver, reported that his team may be nearing a solution. The group previously used a high-energy medical cyclotron to produce Tc-99 from a stable isotope of molybdenum, known as Mo-100. And at the meeting, Schaffer reported that the group has now had success with a GE cyclotron that is the most popular one on the market. Hospitals use medical cyclotrons to generate radioisotopes for medical scans, including fluorine-18 and carbon-11. In the new test, Schaffer's team used a GE cyclotron to fire protons at a Mo-100 target, ejecting a pair of neutrons and generating Tc-99. The demonstration holds out the hope that existing medical cyclotrons could generate enough Tc-99 to fulfill Canada's entire demand, Schaffer says. The group is now working to maximize the amount of Tc-99 produced with this method.
"This is wonderful for Canada," says Robert Atcher, director of the National Isotope Development Center at Los Alamos National Laboratory in New Mexico. But Atcher questions whether the approach will work in the United States. For starters, he says, most Canadians live in metropolitan centers near large hospitals that have cyclotrons. By contrast, the U.S. population is more diffuse, and many outlying hospitals don't have access to a cyclotron. Tc-99 decays so quickly those hospitals would need to be continuously resupplied. In addition, many of the cyclotrons in the United States are not powerful enough to generate Tc-99. Shaffer counters that upgrading cyclotrons is less expensive than building a new nuclear facility, and outlying hospitals already receive regular supplies of fluorine-18 and other short-lived compounds.
Atcher adds that several other groups are exploring alternative non-reactor-based methods for producing Tc-99. It's still too early to say which option will come out ahead, he says. No matter which one does, it should lead to a more stable—and safer—supply of a compound that has become essential to modern medicine.