A threatened shortage of medical isotopes used in cancer treatments has been averted.

A crucial Atomic Energy of Canada reactor at Chalk River, Ont., has restarted after a routine maintenance shutdown ran a couple of days longer than expected.

Dr. Christopher O'Brien of the Ontario Association of Nuclear Medicine says the isotope supply should be completely restored by Thursday.

Meanwhile, he says, there are enough doses available for emergency treatments, so no patients will be affected.

He says, though, the possible shortage is a warning, because the aging Chalk River reactor has to be replaced.

The facility supplies more than half of the medical isotopes used in Canada, United States, Mexico and countries in Europe.

Chalk River's importance gained worldwide attention in 2007 when the reactor was taken offline, causing a worldwide shortage in medical isotopes.