Non-radioactive technology is also used in cancer detection

Doctors have described the shortage of medical nuclear material as "potentially catastrophic".

The Chalk River reactor, in Ontario, was closed two weeks ago for scheduled maintenance which has now been prolonged until early or mid-January.

The reactor supplies two-thirds of the world's medical isotopes to hospitals.

Safety standards

The isotopes are injected as a radioactive dye into patients with cancer or other diseases to allow doctors to take detailed scans.

Thousands of patients in Canada, the United States and other countries have had vital tests postponed because of the shortage.

"Last week, I guess you could describe it as struggling," said Dr Chris O'Brien, president of the Ontario Association of Nuclear Medicine.

"This week it's devastating, and next week potentially catastrophic."

The head of the Canadian Nuclear Safety Commission said the government-run reactor was violating safety standards.

The sudden shutdown has created problems for MDS Nordion, the medical supply company which sells much of Chalk River's output.

The company said it anticipates a return to full production in early to mid-January.

In the meantime, company officials said it was looking for other sources of supply for the radioisotopes.

The isotopes cannot be stockpiled because they have a short shelf life.