Prime Minister Stephen Harper said on June 10 that Canada would be getting out of the business of medical isotope production, as the federal government plans to spin off Atomic Energy of Canada Ltd.'s nuclear reactor business as part of a major restructuring for the company's research facility in Chalk River, Ont.
Canada may not be getting out of isotope production entirely, as a number of scientific and medical facilities across the country begin turning to an older technology, cyclotrons, to produce a different class of isotope than those created in nuclear reactors.
Here we explore how cyclotrons might help lessen our reliance on reactors to produce medical isotopes, which are used for medical imaging and in scans to diagnose illnesses such as cancer and heart ailments.
What is a cyclotron?
A cyclotron is a simple kind of particle accelerator, first developed almost 80 years ago in the United States. Stable, non-radioactive isotopes are accelerated using a combination of high-powered magnets and an alternating voltage applied to the isotopes. The magnets direct the charged particles in a circular path, while the alternating current increasing the energy of the particle each time it crosses a threshold down the middle of the circular path. As a result, the particle gains energy as it spirals down the path of the cyclotron, until it is emitted from the accelerator at high-energies.
Cyclotrons can be used for a variety of physics experiments, but for the purposes of medical imaging, their use is straightforward: the high-energy particles are then slammed into other atoms to produce short-lived positron-emitting isotopes used in positron emission tomography (PET) scans. Isotopes created in nuclear reactors, on the other hand, are used in single-photon emission computerized tomography (SPECT) scans.
How are these isotopes different from the isotopes produced in Chalk River?
A number of different isotopes can be created using cyclotrons, but one thing they all have in common is that, relative to molybdenum-99, the isotope produced in reactors — and technetium-99, a product of molybdenum — is that they are all far less stable.
While Mo-99 has a half-life of 66 hours, a cyclotron isotope like fluorine-18, the most commonly used PET tracer for oncology, has a half-life of about 110 minutes.
A half-life isn't an expiry date, said Dr. Christopher O'Brien, the president of Ontario Association of Nuclear Medicine. But the longer an isotope decays, the less useful it is for imaging purposes. Fluorine-18, for example, would have to be used within three to four hours of being produced before imaging would prove too difficult.
This means isotopes used in PET scanners must be created using on-site cyclotrons, or at the very least cyclotrons located close enough to transport quickly.
The benefit of cyclotron-created isotopes is that when used in PET scanners they produce superior images, said Tom Ruth, a chemistry adjunct professor at the Vancouver-based TRI-University Meson Facility (or TRIUMF), a nuclear research facility that has one of Canada's cyclotrons.
The cyclotron isotopes produce two decay products instead of one for Mo-99, meaning they produce a stronger signal that can more easily be detected using medical imagers. It's this improved imaging, and not the isotope shortage, that is behind a recent move towards in-hospital cyclotron facilities across the country.
Where are the cyclotrons in Canada?
There are eight cyclotron facilities in Canada, one of which, the TRIUMF cyclotron in Vancouver, has five cyclotrons and principally uses them for research.
The others owners are:
- Cross Cancer Institute, Edmonton, Alta.,
- Hamilton Health Sciences, Hamilton, Ont.,
- Ottawa Heart Institute, Ottawa.
- Centre for Addiction and Mental Health, Toronto.
- Montreal Neurological Institute, Montreal.
- University of Sherbrooke Hospital, Sherbrooke, Que.,
- Pharmalogic, a privately owned company in Montreal.
An additional seven cyclotrons are expected to be up and running either this year or in early 2010, located in Vancouver, Winnipeg, Thunder Bay, London, Ont., Halifax, and two more in Toronto.
Together these cyclotrons meet the needs of 25 publicly funded PET scanners across the country, as well as four private facilities with PET scanners, according to a 2007 report produced by the Canadian Agency for Drugs and Technologies in Health, the most recent numbers available.
How much do they cost?
Cyclotrons are expensive for an individual hospital, but relative to the cost of a nuclear facility, their cost is small. Winnipeg's Health Sciences Centre spent $5 million to add a cyclotron this year, just a small part of the medical facility's $150-million expenditure on its Siemens Institute for Advanced Medicine. Other institutions across Canada have built or are building cyclotron facilities at costs ranging from $2.5 million to $6 million. A nuclear reactor, on the other hand, costs about $1 billion to build.
But the cyclotron is only one part of using medical isotopes, since they need to be near PET scanners to be useful. PET scanners can cost another $2 million to $3 million, said Ruth. There is also maintenance and operating costs for the hospitals, said Ruth.
Are cyclotrons capable of replacing reactors?
Ruth said our need for reactor isotopes isn't going to go away anytime soon.
"We're going to need molybdenum-99 and technetium-99 for at least another decade," he said. "Cyclotron-produced isotopes can help, but they can't completely replace the other isotopes because they are so short-lived."
Ruth is working on developing a research network with the other Canadian cyclotron facilities to work to create new isotopes that can be used in PET scanners. He also hopes the government can help fund research into other technologies, such as particle accelerator technology, in which an accelerator fires photons at a relatively stable uranium isotope, uranium 238, to produce isotopes. Scientists have concluded that such accelerators could be built in principle, but research is needed to verify those conclusions before they could become a reality, he said.
In the meantime, he said, cyclotrons have more than proven their worth to the medical community where they are present.
"Anecdotally I know there are some oncologists [doctors who deal with cancerous tumours] who will not perform surgeries unless they've seen a PET scan," he said. "They have become a huge part of the decision making process."