TORONTO -- A review of 3,500 CT scans and mammograms is being conducted at hospitals in Toronto and Mississauga for possible errors due to a "performance issue" with a radiologist.

Trillium Health Partners says the tests were conducted between April 1, 2012 and March 31, 2013 at Mississauga Hospital and Queensway Health Centre.

It says a performance issue with a radiologist who no longer works for the hospitals prompted the review, which will look into the accuracy of the test results.

Trillium said patients and doctors whose scans are involved in the review are being notified, and will be followed up with directly after their tests have been checked.

"We apologize for any concern the news of this review may cause and want our patients and community to know it is being done to ensure the highest quality of care at our hospital," Trillium president and CEO Michelle DiEmanuele said Wednesday night in a release.

An external review will be conducted by a team led by Dr. Brian Yemen, chief of diagnostic imaging at Juravinski Hospital and Cancer Centre and McMaster University Medical Centre.

The review will be expanded as necessary and made public once it is complete, Trillium said.

Phone lines have been set up for patients and doctors concerned about the review of the scans.

"Delivering high quality care and patient safety is our top priority," Trillium chief of staff Dr. Dante Morra said in a release.

Earlier this year, it was revealed that about 1,200 cancer patients in Ontario and New Brunswick had received watered-down treatments, some for as long as a year.

An expert report prepared for the Ontario government called on Health Canada to regulate the preparation of all drug mixtures outside licensed pharmacies.

The report by Dr. Jake Thiessen also urged Ontario to bring in stronger rules for licensed pharmacies.

And in 2011, a Windsor, Ont., surgeon who performed two unnecessary mastectomies was cautioned by Ontario's medical regulatory college.

Dr. Barbara Heartwell became the subject of several investigations after it was revealed she mistakenly removed the healthy breasts of two women at Hotel-Dieu Grace Hospital.

A provincial report into pathology errors largely cleared Heartwell, but uncovered "significant concerns" with pathologist Dr. Olive Williams' work.

After reviewing more than 6,000 reports stretching back to 2003, investigators had moderate or major disagreements with the original diagnosis in 221 of the cases by Williams.