Ontario's governing Liberals say hospitals in the province will soon have more rigorous quality control measures for diagnostic images, such as CT scans.
Health Minister Deb Matthews says the province will be rolling out a "physician peer-review program" in all facilities where diagnostic imaging services are provided.
Her spokeswoman says a team of doctors will review random samples of diagnostic images that have already been assessed by a radiologist.
The move comes after two Toronto-area hospitals discovered that there were possible errors in the reading of more than 3,500 mammograms and CT scans.
Trillium Health Partners were alerted to concerns about one radiologist last March, but only made their worries public more than five months later.
One patient – who has since died – said she had been given a clean bill of health after a CT scan, only to find out later than she had cancer at the time.
After an external review of the more than 3,500 scans at Mississauga Hospital and Queensway Health Centre, Trillium Health said Thursday it found that 11 patients experienced a "clinically significant event."
Eight patients had already been picked up through other testing, but three were identified through the review and are now receiving care, it said.
"We sincerely apologize to patients who experienced a delay in treatment," Trillium president and CEO Michelle DiEmanuele said in a statement.
Trillium said it will bring in a formal peer-review program within its radiology department.
While many hospitals have good quality assurance measures in place, more can be done to strengthen the system, Matthews said.
"Peer review has been found to be an effective method for enhancing safety and accuracy in diagnostic imaging in many jurisdictions around the world," she said.
"Going forward, we will also be looking at additional ways to strengthen health care quality assurance, which may include an accreditation program."
The incidents follows other mistakes in cancer tests and diagnostic procedures across Canada.
In 2012, a sweeping medical study of thousands of mammograms in Quebec found 109 cases of breast cancer that had not been previously diagnosed.
Alberta conducted a system-wide review of medical testing in 2011 following reports about 325 patients were misdiagnosed at three hospitals.
Also in 2011, a report in British Columbia found that poor oversight, among other factors, was to blame in the deaths of three patients and the harming of nine more after they were misdiagnosed by three unqualified radiologists.
And in 2009, a public inquiry in Newfoundland and Labrador determined there had been more than 400 cases of misdiagnosed breast cancer from 1997 to 2005, with some of the patients dying as a result of lack of proper care.