Backgrounder: Producing medical isotopes using X-raysVersion française
On June 2, 2010, the Government of Canada announced the $35 million Non-reactor-based Isotope Supply Contribution Program (NISP) to promote research into alternative methods for producing medical isotopes to address the shortage of technetium-99m (Tc-99m) in Canada due to ongoing difficulties with the National Research Universal (NRU) reactor. The Canadian Light Source led a proposal under the NISP to investigate the technical and economic feasibility of using an electron linear accelerator to produce molybdenum-99 (Mo-99) – the parent isotope of Tc-99m – from the stable isotope, molybdenum-100 (Mo-100).
Isotopes are atoms of the same element with differing numbers of neutrons in their nuclei. Stable isotopes of an element do not change over time. Atoms of unstable isotopes – also called radioisotopes--change into other elements over time through radioactive decay. Radioisotopes are used in many medical imaging and diagnostic procedures, with the isotope Tc-99m being used in 80 percent of tests. In Canada alone, Tc-99m is used in approximately 5500 medical scans a day.
To date, most radioisotopes are produced in a small number of aging nuclear research reactors around the world, as by-products from the fission of highly enriched, weapons grade uranium. In the case of Tc-99m, Mo-99 is collected from the by-products of splitting the uranium atoms, packaged into nuclear pharmaceuticals and shipped to hospitals around the world. The Mo-99, with a half-life of 66 hours, decays into Tc-99m, which has a half-life of 6 hours.
Using nuclear reactors to produce medical isotopes come with a number of challenges. Aging reactors are becoming increasingly unreliable and outages—such as the year-long outage of the NRU reactor at Chalk River – contribute to ongoing shortages. The use of highly enriched uranium is also a major security and proliferation concern, with many nations, including the United States, actively working to eliminate its use in civilian applications. Since half of the Mo-99 decays every 66 hours, much of the resulting Tc-99m ends up being wasted as it decays during shipment from far-flung reactors, to pharmaceutical companies, and finally to hospitals. Isotope-generating reactors also create other by-products besides Mo-99 that persist as long-lived nuclear waste.
How does it work?
The CLS-led project proposes producing Mo-99 using a process called a photoneutron reaction as demonstrated by collaborators at the National Research Council Canada (NRC):
- The linear accelerator accelerates electrons up to close to the speed of light. The high energy electrons collide with a metal filter, producing extremely intense X- rays.
- The X-rays irradiate a target made of Mo-100 metal, with the X-rays removing a single neutron each from a few of the atoms in the metal – making them into Mo-99.
- The molybdenum target containing both the Mo-100 and Mo-99 is then dissolved in a liquid for shipment to hospitals. Mo-99 decays into Tc-99m in the liquid.
- A machine called a radionuclide separator separates the Tc-99m from the solution to be prepared for injection into patients for imaging tests.
- After the Mo-99 has decayed, the remaining Mo-100 in the solution is recovered and recycled into additional targets.
Who is involved in the project?
The project collaborators include the CLS, NRC, the U.S.-based NorthStar Medical Radioisotopes, the University of Ottawa Heart Institute, and the University Health Network affiliated with the University of Toronto. The CLS will design and host the test facility, with design and technical support from NRC. NorthStar will provide the radionuclide separator. Finally, researchers with the University of Ottawa Heart Institute and the University Health Network will be responsible for clinical validation studies of the Tc-99m.
Diagram of the proposed process. An electron beam from a linear accelerator is used to produce high-energy X-rays. X-rays shine on a target consisting of molybdenum-100 (Mo-100) discs. An X-ray strikes the nucleus of a Mo-100 atom, knocking away a neutron to create molybdenum-99 (Mo-99), which decays to become technetium-99m (Tc-99m). A radionuclide separator separates the Tc-99m from the Mo-100 so that it can be injected into patients undergoing medical tests. The Mo-100 can then be recycled into new targets.
What does the project involve?
The project involves installing, licensing and testing a new linear accelerator and equipment to recover isotopes in one of the underground experimental halls originally built as part of the Saskatchewan Accelerator Laboratory – the forerunner of the CLS. This facility would be used to test the technical feasibility of generating and extracting isotopes.
How would this project solve the isotope shortage?
If the project proves to be technically and economically feasible, it is envisioned that ‘accelerator clusters’ in one or more locations in Canada would produce Mo99, which would then be transported to hospitals for recovery of the Tc-99m using a radionuclide separator. It is estimated that two or three such systems would be able to meet Canadian demand.
How does this project solve the other drawbacks of using nuclear reactors to make isotopes?
The Mo-99 – the parent of the Tc-99m – is produced directly from Mo-100 by removing a single neutron from each atom, rather than from the splitting of highly enriched uranium atoms into a number of isotopes, including Mo-99. By taking highly enriched uranium out of the production equation, concerns about security, proliferation and long-lived nuclear waste by- products are eliminated.