A countrywide shortage of radioactive isotopes could become critical following news that the Ontario nuclear reactor that produces them could be shut down until mid-January.

Thousands of patients in Canada, the U.S. and other countries are having their medical tests postponed because of the plant closure, which had initially been scheduled for one week for maintenance repairs.

The government-run Chalk River reactor is the source of more than two-thirds of the world's demand for medical radioactive isotopes, which are used to diagnose cancer and cardiovascular diseases.

Once injected into patients, the radioactive isotopes allow radiologists to zero in on areas of higher radiation and to pinpoint key changes in the body to make accurate diagnoses.

"We have one supplier for a product that is in routine use across the globe and especially in Canada and North America," said Dr. Sandor Demeter, of the Canadian Society for Nuclear Medicine.

"With no contingency in place, if that particular supplier goes down, we're left phenomenally vulnerable."

The supplier, Ottawa-based MDS Nordion, relies on the Chalk River nuclear reactor for its isotopes.

The planned maintenance shutdown of the 50-year-old reactor was supposed to last from Nov. 18 to Nov. 23, but safety problems persist.

The shutdown, now in its third week, could stretch until mid-January, according to the isotopes' distributor.

The radioactive isotopes can't be stockpiled because they have a short shelf life.

Hospitals and pharmaceutical companies are now scrambling to find replacement isotopes.

In many Canadian cities, hospitals are only doing emergency nuclear medicine tests.

Some doctors blame Ottawa

Some doctors blame the federal government for the isotope shortage, saying if Ottawa had taken action years ago to build backup reactors, hospitals worldwide would not have to rely on a single reactor.

During question period at the House of Commons, Health Minister Tony Clement said he's working on a solution.

"There were contingency plans in place, of course, (but) you can't have too much of a contingency plan because the half-life of these isotopes is just three days," Clement said.

"We are working with industry right now and we are getting emergency supplies for emergency procedures and that will continue."

Some Canadian hospitals have contracts with European suppliers and may be able to share their isotopes.

But it's not clear yet if the European suppliers can increase production quickly enough to meet the demand.

With files from the Canadian Press