The Cyclotron and its role in medical research and diagnostics
By: Kim Latimer and Dr. Michael Campbell
A new cyclotron and laboratory are coming to Thunder Bay to enhance patient care and research at Thunder Bay Regional Research Institute (TBRRI) and Thunder Bay Regional Health Sciences Centre (TBRHSC).
So what exactly is a Cyclotron? To explain, we called upon Dr. Michael Campbell, PhD, who is Director of Research Operations at the TBRRI, and adjunct professor of chemistry at Lakehead University.
Q) What is a Cyclotron?
A) A cyclotron is a machine used to make relatively short lived radioisotopes (radioactive atoms) that can be used for medical imaging and research. The cyclotron does this by taking hydride atoms (these are the same as the hydrogens that make up water except they have a negative charge) and accelerating them to very high speed. When they have enough energy we send them into a target where a reaction takes place. In much the same way that the cue ball can transfer its energy to a pool ball and send it on its way, the now positively charged hydrogen atom (proton) is able to knock out a neutron in the target material and produce a new element. The new element that is produced can be radioactive and be used for patient care or research.
The cyclotron that Thunder Bay is getting is a medium energy cyclotron and at 24 MeV (mega electronvolts) is able to accelerate a hydride atom to approximately 68,000 km/second or little under a quarter of the speed of light in less than a second. While that sounds fast, no one has to worry about high speed atoms flying overhead. The hydride is only able to move that fast in the high vacuum of the cyclotron. Outside the cyclotron the particles wouldn’t even be able to travel a meter.
Q) What will the products of the cyclotron be used for?
A) The major isotope used at the Health Sciences Centre will be fluorine-18. What makes fluoride 18 special is that it is a type of radioactive isotope that when it decays it produces something called positions. These are the key to Positron Emission Tomography or PET scans used daily around the world for cancer diagnosis and treatment planning. The fluorine-18 can be used directly for such things as bone scans or most commonly attached to a sugar to make a compound called FDG for cancer diagnostic imaging. FDG works because cancer cells require a lot of energy and they consume this radioactive sugar faster than other healthy cells, because of this the cancer cells will show up on the scanner this helps with treatment planning and staging.
With most cyclotrons this would be pretty much the extent of the isotopes produced. However because of the energy of the cyclotron we are getting, we will be able to produce a wider variety of isotopes of interest for medical imaging such as Iodine-123, Gallium-67, and even Technetium-99m. Technetium-99m was the isotope that was in such short supply when the Chalk River NRU reactor was shut down.
Q) Why build a cyclotron here versus just bringing the isotopes in from another site?
A) All radioactive isotopes have something called the half-life. This is the amount of time that it takes for half of the material to decay into something else. So if you start with a certain amount of material after 1 half-life you would only have 50% remaining, after 2 half-lives 25% and after 3 half-lives 12.5% and so on. While this isn’t a problem for radioisotopes that have a long half-life, many of the isotopes used in PET such as the fluorine-18 used to make FDG have a relatively short half life (110 minutes). Currently, TBRHSC gets its FDG from southern Ontario and by the time it is prepared, tested, driven to the airport in Toronto, and sent here by plane, close to 3 half-lives have been lost. That means that in order to run scans on 4 patients in Thunder Bay, the facility in southern Ontario has to produce and ship enough FDG that could otherwise scan 32 patients. By having a cyclotron and radiopharmacy on-site we will be able to reduce the cost as well as increase the number of scans available to our patients on a given day. Additionally, we will be able to make use of other isotopes that have half-lives that are too short and can’t be shipped in.
Q) Is a cyclotron safe?
A) A cyclotron is a machine used to produce radioactive materials, so it requires licensing by the Canadian Nuclear Safety Commission (CNSC). Cyclotrons are not new technology and the CNSC has extensive experience with cyclotrons from the other facilities in Canada. It takes worker and public safety very seriously.
The facility in Thunder Bay will be designed with multiple levels of shielding, protection, and monitoring to ensure safe operation. Before construction ever begins we must apply to the CNSC for a license to construct, in this application they will look at the design and shielding calculations for the facility to ensure the cyclotron can be used safely. This is combined with regular monitoring, yearly annual compliance reports, and regular license renewals to ensure compliance with CNSC regulations. All exposure levels to staff working in the facility, outside the facility, as well as guests must be well below the CNSC allowable limits.
As mentioned above most of the isotopes have a relatively short half-life and rapidly decay into products that are no longer radioactive. Another thing that makes production of isotopes on a cyclotron safer is the reaction scale is quite small. The production of fluorine-18, for example, uses approximately 5ml of oxygen-18 enriched water (less than the volume of a coffee creamer).
Q) What is the difference between a cyclotron and nuclear reactor?
A) There a several differences between a cyclotron and a nuclear reactor. For the production of medical isotopes, the main difference is the starting material. In the case of a cyclotron, the starting materials are generally stable, non-radioactive materials such as oxygen-18 enriched water or nitrogen gas. In the case of a reactor, the raw material is typically uranium which is split to give different radioactive products that are separated and purified for use. Since cyclotrons don’t use starting materials like uranium they also don’t produce the same long-life radioactive waste as nuclear reactors.
Another difference is the source of power. Reactors get their power from the fission of uranium which produces heat and neutrons to keep a chain reaction going, a cyclotron gets its power from electricity. A reactor, even when it is not producing isotopes, must maintain cooling and can take a long time to shut down compared with a cyclotron that can be shut down by turning off the power.
There will always be a need for reactors to produce isotopes that either can’t be produced on a cyclotron, or in sufficient quantity for worldwide distribution. The cyclotron, however, does offer a good, flexible option for the production of a variety of isotopes on a regional scale, and most importantly, helps provide care for our patients in Northwestern Ontario.