A health crisis is currently underway, with barely any coverage in the American media. Over half of the world’s supply of molybdenum-99, an isotope used to generate radiopharmaceuticals for medical imaging procedures, is created in a single nuclear reactor in Chalk River, Ontario. The reactor shut down in November 2007 due to safety concerns, and has closed twice since then for needed repairs. On May 15 of this year a radioactive water leak forced it to shutdown again, and this time there is speculation that it may not reopen.
Molybdenum-99 (Mo-99) is used as a generator for technetium-99m (Tc-99m) which is a tracer for imaging procedures such as Single Photon Emission Computed Tomography or SPECT. A generator is a radioactive isotope that can be easily and safely stored but decays into another isotope. In the case of Tc-99m, the half life is only 6 hours. This means that if it is to be used at a hospital for an imaging procedure, then it must be produced on site. Molybdenum-99 has a half life of 66 hours. This means it can produced elsewhere and shipped to the hospital. The Tc-99m can be chemically separated when needed and given to the patient. Colloquially, the generator is called a “cow” and the process of removing the daughter isotope is called “milking”.
Molybdenum-99 is made using neutrons from a nuclear reactor. When these neutrons strike uranium-235 nuclei, the nuclei undergo fission. Around 6% of the time, this fission results in an atom of Mo-99. This process requires large amounts of highly enriched uranium, with all the nuclear weapons proliferation issues that represents. Other processes involving neutron capture and photoneutron production are being researched. These would have the advantage of being able to use low-enriched uranium. Presently, though, all Mo-99 production is done with highly enriched uranium.
After the Tc-99m is produced, it can be attached to a molecule that has properties desired for the imaging procedure. This creates a substance that is called a tracer. For example, to image cardiac bloodflow, a tracer called 99mTc-tetrofosmin or 99mTc-sestamibi is injected into the body. It then collects in the heart, with more of it collecting where blood flow is strongest. As the Tc-99m decays, it gives off a gamma ray photon that can be detected by a camera outside of the patient. By analyzing the amount of photons detected as the camera moves around the patient, the three dimensional distribution of Tc-99m can be reconstructed. Any area of low blood flow or cardiac defect can then be seen.
A large number of nuclear medicine imaging studies rely on Tc-99m. In 2008 of 22.5 million doses of diagnostic radiopharmaceuticals were given to patients in the United States, 18.5 million or 82% of these were Tc-99m. Any disruption in the supply would therefore have a dramatic effect on diagnostic imaging. When the Chalk River reactor closed in 2007, many tests had to be postponed. Exacerbating the problem this time is that three of the other four reactors that produce Mo-99 are currently down for maintenance. The future of the reactor is in doubt as there are conflicting reports from the Canadian government. A CTV report states that some officials say the reactor will be down for three months and others say that the reactor cannot be repaired. In addition, the Canadian government has announced plans to privatize the nuclear reactor business of Atomic Energy of Canada Ltd., a Canadian Crown, or government owned, corporation.
As my headline suggests, the real problem here is not the Canadian government’s spotty record at operating the Chalk River plant. Instead it seems incomprehensible that a radioisotope that is so vital to so many procedures is only available in North America from a single source. The cost of building and operating a nuclear reactor and handling highly enriched uranium is just too large compared to the potential profit from radiopharmaceuticals, I suppose. A report from the Society of Nuclear Medicine identified several reactors that could be converted to Mo-99 production, but all of them had technical and regulatory hurdles to overcome. Even if the effort were to start today,production from any source would be at least three to four years away. Private companies have also announced plans to develop Mo-99 production capability. That effort is also years away from fruition.
The extent of the shortage of Mo-99 is still unknown, but all signs point toward a significant disruption of medical imaging procedures in the United States. We need to make it a priority in the United States to have at least two sources of medical isotopes in production as soon as possible. Until then, we are at the mercy of equipment failures at reactors all over the world.
