Medical isotopes

They're the backbone of nuclear medicine. Medical isotopes — tiny radioactive particles that can be injected into the body — have become the standard treatment for some cancers. They've also brought medical imaging to new levels.

And most of the world's supply is produced in Canada.

The National Research Universal reactor went fully online at Chalk River, Ont., on Nov. 3, 1957. It has been used for scientific research, including the development of nuclear medicine. It remains the biggest single source in the world of the isotope cobalt-60, which has been used in cancer treatment for more than half a century.

The reactor produces enough isotopes to treat more than 76,000 people a day — more than 20 million a year.

How are medical isotopes useful in diagnosing illness?

Put simply, medical isotopes give off energy that can be detected by imaging equipment. When isotopes are injected into your body, a doctor can — for example — get a clear picture of how your heart is working. The doctor can see whether you're a heart attack waiting to happen. They'll see enough to know whether you should go straight to the hospital for bypass surgery.

The isotopes provide far more information than an ultrasound. They make bone scans far more effective than X-rays. In a bone scan, radioactive material is injected into a vein in the arm. The material travels through the bloodstream and eventually settles in the bones. This will give doctors information on cell activity from which they can tell if you have stress factures in your feet or whether the cancer in another part of your body has spread to the bones. Bone scans can detect problems days or even months before X-rays.

How are medical isotopes used in cancer treatment?

Modern radiation therapy was pioneered in Canada in 1951 in hospitals in Ontario and Saskatchewan. Cobalt-60 was used in a treatment that allowed medical technicians to target a specific part of the body.

The energy given off by medical isotopes is effective at destroying diseased cells. When cancer cells are targeted and destroyed, healthy tissue is left alone.

Are there different types of medical isotope treatments?

Besides standard radiation therapy, there are three common types of treatment using medical isotopes.

Brachytherapy is a form of radiation therapy where radioactive isotopes in the form of small pellets (called seeds) are inserted into cancerous tumours to destroy cancer cells while reducing the exposure of healthy tissue to radiation. It is currently approved for treatment of prostate cancer and cancers of the head and neck. There are also studies underway to see whether it can be used in the treatment of lung cancer.

In radioimmunotherapy (RIT), doctors inject antibodies that have isotopes attached. The antibodies (called monoclonal antibodies) flow through the bloodstream and deliver the radioactivity by seeking out and latching onto proteins on the cancerous cells. Again, it allows doctors to spare healthy tissue while targeting diseased cells. RIT is being studied in several cancers, but has shown the most promise in the treatment of blood cell cancers, such as leukemia and lymphoma. It is also being looked at for treatment of prostate, colorectal and pancreatic cancers, and soft tissue sarcomas.

Medical isotopes can also be paired with carriers that are attracted to certain parts of the body. For instance, chemical phosphonates are naturally attracted to the bone. Delivering medical isotopes to the bone by attaching them to phosphonates is now being used to treat pain associated with cancer that has spread to the bone. As well, iodine has been used for thyroid treatment for years because it is naturally attracted to the thyroid. The medical isotope iodine-131 is used to treat thyroid cancer.

What are some of the challenges in using medical isotopes?

One of the key problems is that many isotopes lose their radioactivity very quickly so they cannot be stockpiled. Cobalt-60 has a half-life of 5.26 years. However, molybdenum-99 has a half-life of 66 hours, which means it loses half its radioactivity in just under three days — and half of what's left in another three days. That's critical because molybdenum-99 is used to produce technetium-99, which is used for eight out of every 10 nuclear medicine procedures.

Iodine-131 has a half-life of 8.02 days while iodine-125 loses half its radioactivity every 59.4 days.

The short half-lives of key isotopes means they have to be delivered quickly.

What's happening to ensure stable production of medical isotopes?

The National Research Universal reactor at Chalk River was scheduled to be decommissioned in 2005 and be replaced by two new reactors called Maple 1 and Maple 2. Those reactors were initially supposed to be completed in early years of this decade. Their only job will be to produce medical isotopes and they will be capable of producing more than the current world demand for molybdenum-99, iodine-131, iodine-125 and xenon-133. The reactors will be the first in the world dedicated solely to the production of medical isotopes.

But until they are ready, the NRU reactor will continue to produce most of the world's medical isotopes. The reactor was shut down for scheduled maintenance on Nov. 18, 2007. However, the shutdown was extended on Dec. 4 and production was not expected to return to normal until early to mid-January 2008.

That has led to the delay or cancellation of procedures at some hospitals in Canada and around the world.

There are other reactors that can produce medical isotopes in Canada. For instance, McMaster University in Hamilton, Ont., has operated a small reactor for 48 years. It's capable of producing a few medical isotopes including iodine-125, but not enough to pick up the slack caused by the extended shutdown of the NRU reactor.