NEW YORK - Thousands of patients are facing delays in crucial medical tests due to a shortage of a radioactive substance used in those examinations - all because of the shutdown of one nuclear reactor in Canada.

The substance is used in at least 15 million medical scans a year in the United States, by one estimate. They diagnose and assess conditions ranging from cancer to heart, bone and kidney diseases.

And the tests are often crucial for guiding therapy, telling a doctor whether a woman's breast cancer has invaded her bones, for example.

Over the last few days, hospitals began facing shortages of a radioactive substance called technitium-99 that is injected into patients for these body scans.

That has forced them to cut back on the procedures.

"Many, many hospitals are working at about 20 to 30 percent of capacity," estimated Sandy McEwan, president of the Society of Nuclear Medicine, based in Reston, Va.

The shortage seemed to be hitting parts of Canada hard.

Ontario is down to half its normal scan capacity, meaning 1,000 patients a day are having tests postponed.

While doctors can often turn to other tests, these can be more complicated and more awkward to interpret.

The cause of all this is the unexpectedly long shutdown of a nuclear reactor in Chalk River, Ontario. The 50-year-old reactor is North America's biggest source of the isotope that makes technitium.

The owner of the reactor, Atomic Energy of Canada Ltd., shut it down Nov. 18 for what was supposed to be five days of routine maintenance but then decided to do more work. It likely will be working again by the end of December and almost surely by late January, the company says.

Meanwhile, the closure stopped the reactor's output of a radioactive substance called molybdenum-99, which is processed and packaged into canisters that are sold to big hospitals and specialized pharmacies. These cylinders are "milked" for their technitium-99, which is prepared for use in the medical scans.

Since the technitium supply from each cylinder eventually peters out, the cylinders have to be regularly replaced. That's when the effect of the reactor shutdown shows up.

Companies that make these cylinders say they're working with molybdenum suppliers in Europe and South Africa to ease the shortage.