OTTAWA - Medical treatments are being delayed or deferred at hospitals worldwide because of the extended shutdown of a Canadian reactor.

The reactor, the Atomic Energy of Canada reactor at Chalk River, Ontario, near Ottawa, is North America's only source of the base isotope for technetium-99, a workhorse of modern medical diagnostic systems.

It is injected into patients 20 million times a year in the United States to create images used in the diagnosis and treatment of a wide variety of illnesses including heart ailments, cancers and gallbladder problems.

Supporters of restarting Hanford's Fast Flux Test Facility have warned that the United States was too dependent on the aging Canadian reactor in their so far unsuccessful push to restart the Hanford reactor to produce medical isotopes, among other uses.

The Canadian reactor closed on Nov. 18 for maintenance. It was scheduled to open five days later but remained closed "to complete the installation of safety-related equipment," the company said.

On Wednesday, Atomic Energy's wholesaler, MDS Nordion, said it did not expect full production to resume until mid-January.

Because the isotopes created by the reactor decay rapidly, they cannot be stockpiled, which is leading to growing shortages of the material at medical centers.

Adding to the problem is the fact that the Atomic Energy reactor produces 50 percent to 80 percent of the world's supply of molybdenum-99, the isotope that breaks down into technetium-99.

The shortfall has renewed decades-old calls for the United States to develop its own medical isotope reactors rather than continuing to rely on imported products from a limited number of producers.

"This is a bad news story in every sense of the word," said Dr. Alexander McEwan, the president of the Society of Nuclear Medicine in Reston, Va. "It means patients are going to suffer. People are going to look at this and say, 'Why are we so reliant on a single supplier?' "

Dr. Henry Royal, a professor of radiology at Washington University in St. Louis, added: "The fundamental problem is that the supply of radiopharmaceuticals is very fragile because we rely on foreign imports, which we have no control over."

Several years ago, government-owned Atomic Energy sold its wholesale distribution and sales business to MDS Nordion, an Ottawa-based company that is owned by MDS, the large Canadian medical services company.

Technetium-99 has a shelf life of six hours, making it impractical to ship over any distance. In the United States, hospitals usually buy specialized containers of molybdenum-99 known as generators.

Those devices, which mostly are sold by Bristol-Myers Squibb and Mallinckrodt, a unit of Covidien, use a chemical process to separate the technetium-99 just before it is needed for patient tests.

Like disposable flashlight batteries, however, the generators eventually run down. The Atomic Energy reactor shutdown has left some hospitals unable to find replacements.

Hospitals affiliated with Yale University have been able to fill the gap by buying technetium-99 from an outside laboratory, Dr. James Frost, a professor of diagnostic radiology, said.

The shortage, however, already is creating after-hours problems for emergency rooms at Yale and Johns Hopkins University, where Frost also holds an academic posting.

With the private labs closed at night and the in-house isotope generators not working, some patients have to wait until morning for treatment, Frost said.

While the effect of that is mostly inconvenience for both the patients and hospitals, Frost said that lack of after-hour diagnostic isotopes was potentially dangerous for a small number of patients with certain conditions.

In a statement, Bristol-Myers Squibb said that "MDS Nordion has assured us, and its other customers, that it is working closely with their backup supply network to minimize the impact of the reactor shutdown on its customers."

But the statement added: "Alternative sources of molybdenum available worldwide are limited."

MDS Nordion declined interview requests, and Covidien said it was unable to discuss its supply situation.

A report released by the Canadian Nuclear Safety Commission indicated that the reactor was shut down after its inspectors discovered that an emergency power supply to an important system was not connected.