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The Canadian processor and distributor of much of the world's medical isotopes is scrambling to secure suppliers in South Africa and Europe as countless tests for cancer and other diseases have been put on hold by the indefinite shutdown of an aging reactor in Chalk River, Ont.

"We're finding out on a day-to-day basis whether we can offer service the next day," said Christopher O'Brien, president of the Ontario Association of Nuclear Medicine, who said the shortage had forced him to turn down a request yesterday to scan for a blood clot in a lung.

"This is beginning to remind me of my time in Uganda, when we had to decide who received medical care because of shortages. We are approaching an East African level of care here in Canada."

MDS Nordion was in talks yesterday with suppliers in the hopes of getting emergency supplies of molybdenum from which technetium is derived. The radioactive substance used in nuclear imaging to help doctors determine treatment for patients with serious diseases.

But spokeswoman Tamra Benjamin was not optimistic that the backup suppliers would be able to mitigate the impact caused by the temporary shutdown of the half-century-old reactor in Ontario, which is the source of more than two-thirds of the world's demand for medical isotopes.

"Unfortunately, other commercial production reactors cannot make up the supply requirements and there will be shortages," she said. "We empathize with the impact this supply disruption is having our doctors, patients and our customers."

Ms. Benjamin declined to list the potential suppliers.

The federally owned Atomic Energy of Canada Ltd. reactor was supposed to be shut down for five days of safety-related maintenance. But it has now been out of commission for more than two weeks, and there's speculation that it may not be operational until next month.

"It's a top-priority item and we're going as fast as we can. But we're not releasing a schedule," said Jeremy Whitlock, a scientist at AECL.

Health Canada officials were closeted in meetings yesterday, dealing with the issue.

A spokeswoman for Health Minister Tony Clement said there is a consensus among all parties to expedite the process of approving other suppliers. "Health Canada officials and the minister have made a commitment to fast-track imports and alternative sources," Laryssa Waler said in an e-mail.

WHAT IS AN ISOTOPE?

An isotope used to "label" body parts during a nuclear scan, technetium-99m, is a second-generation byproduct of nuclear fission.

Fission reactors such as the one in Chalk River split uranium by bombarding it with neutrons. One part of the result is the isotope molybdenum-99, a radioactive variety of the element molybdenum.

Purified by the company MDS Nordion, the resulting substance is the source for the isotope used in the bulk of nuclear scans.

But molybdenum-99 is unstable and begins to decay immediately, having a half-life of only 66 hours.

This means that it loses half of its radioactivity in that time, and half of the resultant radioactivity in each subsequent 66-hour period.

The decaying molybdenum-99 is packaged by MDS Nordion into containers known as generators and then delivered to hospitals in North America and around the world. In those generators - known colloquially as technetium cows - it decays and produces the isotope technetium-99m, crucial for nuclear imaging.

This isotope is removed from the generator in a process known as "milking the cow," said Jeremy Whitlock, a scientist at Atomic Energy of Canada.

Technetium-99m has a half-life of only six hours and Mr. Whitlock described it as similar to electricity.

"You can't store it. You make it when it's used."

The short half-life means that nearly 94 per cent of the isotope has decayed into non-radioactive technetium after the first 24 hours. This keeps total patient radiation exposure low, but also means a regular supply is needed.

"Whenever we need it, we literally take a small vial [of saline solution] and pass it through the generator and technetium comes out the other side," explained radiopharmacist Doug Abrams.

"We take the radioactivity and inject it into the patient."

Caroline Alphonso, Oliver Moore

WHAT IS AN ISOTOPE'S MEDICAL USE?

Isotopes are mildly radioactive substances used to "label" a certain part of the body. That area then gives off gamma rays that are read by a specialized camera. The picture constructed by the gamma camera offers a glimpse of whether the body's inner workings are functioning properly.

Halifax doctor Andrew Ross said the scans are used to examine bones and organs. A scan might turn up a kidney blockage, for example, or indicate problems with liver function. They are also used on the brains of Alzheimer's patients, Dr. Ross added, and can spot a bone tumour well before it would show up on an X-ray.

The imaging process, which involves technetium-99m being ingested or injected, is essentially the reverse of an X-ray, in which radiation is applied from outside the body. Instead of looking for changes in anatomy, the way an X-ray would, the nuclear scan assesses whether the body's functioning has changed in a way that indicates the presence of disease.

Bob Bell, president of the three-hospital University Health Network in Toronto, said they normally do such scans up to 100 times daily. Most tests are for elective surgeries, he said, but there are some urgent needs. In particular, the radioactive substance is used for sentinel node biopsy in cancer surgery, for detection of occult gut bleeding sites and for staging cancer patients prior to curative surgery.

Radiopharmacist Doug Abrams said an estimated 1.2 to 1.5 million nuclear scans are done annually in this country.

According to Atomic Energy of Canada Ltd., technetium-99m is used 50,000 times a day in hospitals and medical facilities around North America. But, with the Chalk River shutdown putting a crimp on supplies, procedures have been slashed at hospitals around the world. The CSNM warned in a statement yesterday that more than 10,000 Canadian treatments will be delayed for every week this supply disruption continues.

Oliver Moore, Caroline Alphonso

HOW DID CHALK RIVER COME TO SUPPLY ISOTOPES?

When the National Research Universal reactor was built in Chalk River, Ont., in 1957, part of the goal was to have it make isotopes.

Another reactor at the site, which started running in 1947, produced the first cobalt-60 used in cancer therapy.

Jeremy Whitlock, a scientist at Atomic Energy of Canada, said officials were pleasantly surprised to find this reactor producing medically used molybdenum-99, an isotope from which technetium-99m is derived.

The latter is injected into patients, and allows radiologists to zero in on areas of higher radiation and to pinpoint key changes in the body to make accurate diagnoses.

Unlike other reactors, Mr. Whitlock said, the NRU has a lot of infrastructure built in, especially in the processing line to extract the molybdenum.

Doug Abrams, an Edmonton-based radiopharmacist and president of the Canadian Society of Nuclear Medicine, said that all fission reactors produce molybdenum but few are set up for it to be retrieved for medical use.

"As I understand it, the problem is getting that fission product out," he said.

"You have to design the reactor specifically so you can get that product out."

Tamra Benjamin, a spokeswoman with MDS Nordion, said there are only a handful of reactors that can be used as a source for nuclear-imaging isotopes.

The aging reactor in Chalk River was to have been buttressed by two new reactors.

Together they would be able to supply the entire world's demand for medical isotopes, according to a 2002 statement from MDS Nordion. But the new reactors are behind schedule and over budget, and the world is now seeing the cost of relying too much on a single source.

Caroline Alphonso, Oliver Moore