NEW YORK — Thousands of patients are facing delays in crucial medical tests because of 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. Those scans are used to diagnose and assess a wide variety of conditions including cancer, heart disease and bone or kidney illnesses.

They are often crucial for guiding therapy, telling a doctor whether a woman’s breast cancer has invaded her bones, for example.

But over the past few days, many hospitals began facing a shortage of a radioactive substance called technitium-99 that is injected into patients to do these body scans. And that has forced them to cut back on the procedures.

“Many, many hospitals are working at about 20 to 30 percent of capacity” in doing the scans in the United States and Canada, estimated Dr. Sandy McEwan, president of the Society of Nuclear Medicine, based in Reston, Va. He said he didn’t know how many scans had been postponed.

The shortage seemed to be hitting parts of Canada hardest. Dr. Chris O’Brien, president of the Ontario Association of Nuclear Medicine, put it this way to Canadian Press: “Last week, I guess you could describe it as struggling. This week it’s devastating, and next week potentially catastrophic.”

Ontario, for example, is now down to about half of its normal scan capacity, meaning about 1,000 patients a day are having their tests put off, he said.

“We don’t know where to put these people,” O’Brien said.

The Canadian Society of Nuclear Medicine estimates the shortage will cause delays in treatment for 50,000 Canadians each month that services are reduced.

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

What’s the cause of all this

It’s 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 radioactive 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. However, the company decided to keep it closed to do more work. The reactor will probably be working again by the end of December and almost surely by the end of January, the company says.

But in the meantime, the shutdown 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 then 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 Chalk River reactor shutdown shows up.

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

But in the meantime, the shortage has affected places like Caritas Holy Family Hospital in Methuen, Mass. Spokeswoman Danielle Perry said the 270-bed hospital has had to delay a few tests because of the shortage.

“We’ve already started to preserve our supply. We’re giving priority to those patients with more urgent or critical needs,” she said.

Larger hospitals are watching their supplies closely.

“We are going day by day,” said Gopal Saha, director of nuclear chemistry and pharmacy at the Cleveland Clinic, which gets two canisters a week. No scans have been canceled there yet, he said Friday morning.

“I can’t tell you what’s going to happen tomorrow. It’s clearly not a good situation” said Dr. Philip Alderson of New York-Presbyterian Hospital.

“I’m a little nervous. I’m concerned about where this is headed if this doesn’t get resolved relatively quickly.”

Alderson said his department learned Thursday that it would no longer get two molybdenum canisters a week, but instead will have to buy technitium dose-by-dose. That could pose a logistical problem if a patient suddenly needs a scan, he said.

“I’m worried about ... the patient who comes to the emergency room at 3 o’clock this afternoon with chest pain” and needs a lung scan to look for a clot, he said. “Will we be able to get that dose”

McEwan said the nuclear medicine society has long pushed for the United States to build its own reactor to produce medical materials. That hasn’t happened for a variety of reasons, including cost, he said. He called that “shortsighted.”

The society is now working with federal regulators to expand the possibilities for using alternative tests, he said, and hospitals are sharing technitium-99 the best they can.

And what should patients think of all this

“They need to talk to their doctors about the best alternative tests,” McEwan said. And “they need to be assured that the whole industry is looking at alternative supply so they can resolve the issue as quickly as possible.”

— AP writers Charmaine Noronha in Toronto and Stephanie Nano in New York contributed to this report.