TORONTO - The health of thousands of Canadian and U.S. patients could be put in jeopardy by a critical shortage of radioactive isotopes over the next few weeks, nuclear medicine experts say.

The Society of Nuclear Medicine (SNM) says there will be a severe worldwide shortage of Technetium-99 used in imaging tests for cardiac, cancer and other patients for a two-to three-week period starting Sunday.

That's because the three nuclear reactors that produce the radioactive substance Molybdenum-99 -needed to generate Technetium-99 - will be going down for much-needed maintenance and refuelling, said SNM spokesman Dr. Robert Atcher.

Canada's NRU reactor at Chalk River, Ont., had supplied a third of the world's medical isotopes until it was shut down last May to repair a pinprick-sized radioactive water leak. Atomic Energy of Canada Ltd. had predicted the reactor would be back up by spring, but Atcher said there's been no word on when it will begin operating again.

At the same time, the Netherlands's Petten reactor has been taken off-line, leaving just three reactors in Belgium, France and South Africa to carry the load of producing enough Molybdenum-99 to supply the world market for Technetium-99.

"Those other three reactors, the ones that are operating, have required outages for maintenance, for fuel and so on," Atcher, director of U.S. National Isotope Development Center, said Thursday from Los Alamos, N.M.

"And we knew as of December that there were going to be two periods when NRU and Petten were off-line, when we were going to have severe problems. And this is the first one."

During the critical period, from Sunday to April 11, the SNM estimates that thousands of patients will not be able to get the diagnostic procedures they need.

"We're predicting that there will be a minimum of 30,000 patients a day that are not going to be able to be imaged because we won't have the Technetium available," he said. "And the alternative radioisotopes, we haven't been able to ramp up supply sufficiently."

Dr. Christopher O'Brien, head of the Ontario Association of Nuclear Medicine, says Canada will also be "in dire straits."

"The hope is that we will still be able to maintain emergency services, but we all anticipate that our activity levels will only be known 24 hours to 48 hours prior to a patient being booked," he said. "So for those 2 1/2 weeks we're looking at it will be a struggle. It will be a strain on the Canadian system.

"Hopefully our urgent cases will be able to be taken care of."