Radioisotopes: supply and demand
8/8/2008
Nuclear medicine is one of the fastest growing segments in the healthcare industry today and the demand for medical isotopes is increasing in tandem. Unfortunately the supply of these isotopes may not always keep up with the demand. Take molybdenum-99 (Mo-99), for example. Mo-99 is the precursor to technetium-99m, a short-lived radioisotope that's used in 80-85% of all diagnostic nuclear-medicine procedures worldwide. Its production, however, conventionally demands the use of a large nuclear reactor.
And herein lies the problem - nuclear reactors are inherently burdened with overhead, infrastructure and regulatory expenses. In the US, for example, there's not one domestic reactor currently producing Mo-99. Instead, 80% of the country's supply of this key isotope comes from a single source - the National Research Universal reactor in Ontario, Canada. And when this reactor shutdown unexpectedly last December, it brought into stark reality the vulnerability of
such a limited supply line.
In an attempt to address this issue, radioisotope specialist Advanced Medical Isotope Corporation (AMIC; Kennewick, WA) has teamed up with scientists at the University of Missouri (Columbia, MO) to develop a way to produce isotopes like Mo-99 without needing a nuclear reactor. The new device will be based on a sub-critical system invented at the University of Missouri, which can generate clinical quantities of radioisotopes from a device the size of a commercial cyclotron.
The system works by bombarding a tank of heavy water with high-energy gamma rays, created by firing an electron beam at a tantalum target. The gamma rays cause the deuterium nuclei to disassociate, resulting in a surfeit of high-energy (MeV) neutrons within the tank. These neutrons then react with a uranium target situated in the tank to produce Mo-99.
The actual creation of the Mo-99 - via fission of U-235 - works in the same as in a regular nuclear reactor. "The unique part of the device lies in the design of the heavy-water system and recovering the Mo-99," explained AMIC's chief science officer Robert Schenter."The purpose of the heavy water tank is to slow the neutrons down to energies such as those found in a thermal reactor. The system's major advantages are its compact size - it can fit in a room - and its much
lower cost."
While the development team is focusing on Mo-99 production, various other medical isotopes can be created by simply switching the target material. Schenter cites dysprosium-165 - a shortlived beta-emitter used for treating arthritis - as a prime example. "Dysprosium-165 only has a 2.3-hour half-life," he explained. "A compact machine like this - which is more acceptable than a nuclear reactor - could be located in any big city to supply local hospitals with this isotope."
Model approach
AMIC has employed a range of computer simulations to predict the performance of the new production device. Calculations indicate that the system could create at least 500 Ci of Mo-99 in one week of operation (roughly one-tenth of the current demand in the US), with the potential for even higher production levels.
"We've got to do more extensive rounds of computer modelling to find the most promising mix of uranium and heavy water, as well as to determine the optimal size and shape of the tank to maximise neutron production," James Katzaroff, CEO and chairman of AMIC, told medicalphysicsweb. "We then need to build a prototype to confirm that the system operates in the real world the way that the computer models predict."
With plans to build such a prototype next January, Katzaroff reckons that the device could be operational within three years. This time would allow for prototype production, system debugging and implementation, plus the quality-assurance programmes needed for the production of radioisotopes used in humans.
"There are a number of proven methods for extracting Mo-99 from the heavy water and we still need to determine which one works best," he added. "But there's no new development work necessary, in the sense of any scientific breakthroughs, to make this work and all the components will be off-the-shelf."
If the implementation is successful, AMIC could find the market ready and waiting for such a radioisotope production device. According to projections by the National Academy of Sciences, the US currently requires at least 5000 Ci of Mo-99 each week, with demand expected to grow at a minimum of 5-10% in the coming years.
"The proposed device creates the potential to rapidly move into high-demand markets with a network of radioisotope production facilities. There's a global demand for medical isotopes and this may be one of the primary ways to alleviate such demand," Katzaroff explained. "We also see this as something that's ideal for developing countries. Bringing this sort of technology globally at a reasonable price-point has a lot of appeal to us."
By Tami Freeman, Editor
MedicalPhysicsWeb
