"The concerns about the radiologist were discovered by our Chief of Diagnostic Imaging on March 28, 2013, who acted immediately to reduce risk to patients," Trillium Health Partners spokeswoman Suset Silva said in an email to The Canadian Press.
"The radiologist did not review any further CT scans or mammography reports, and the chief proactively launched an internal investigation into the radiologist's practice."
Trillium announced Wednesday it had uncovered a "performance issue" with a veteran radiologist concerning diagnostic tests conducted between April 1, 2012, and March 31, 2013, at two of its three sites, Mississauga Hospital and Queensway Health Centre in Toronto.
Trillium did not inform the Ministry of Health about the situation with the possibly misdiagnosed test results until late August, but said it removed the radiologist's hospital privileges after a two month internal probe.
"Immediately after obtaining the results of the investigation, the hospital initiated a process to have the physician's privileges restricted by its board of directors," said Silva.
In late May, the Trillium board restricted the radiologist's privileges and notified the College of Physicians and Surgeons of the decision June 3, she added.
One patient, Houda Rafle, 28, told Toronto television station CP24 that she was given a clean bill of health after a CT scan in March, only to find out recently that she did indeed have cancer at the time, and it has gotten worse.
"The tumour was present in March, and unfortunately because there was a six month duration it had now spread to my lungs and is now stage 4," said Rafle.
The misdiagnosis and the fact the untreated cancer has spread triggered a wide range of emotions when she got the news, added Rafle.
"It's frustration, disappointment, just devastated, and when I heard this news the one thing I knew was I have to get through this, I have to no option but to recover."
The Ontario government moved quickly to assure people the problem was caught with normal internal hospital procedures, and they should have total confidence in the health care system.
"I am interested in learning about what other provinces are doing, but I think it's important to acknowledge our system is quite different, so that quality oversight is happening in our hospitals," said Health Minister Deb Matthews.
Receiving such a letter from a hospital can cause a lot of stress and anxiety, added Matthews.
"I know that they are very worried," she said. "I understand that, and that's why I'm pleased that the hospital is moving as quickly as it can to review each and every case."
Ontario's New Democrats said the Trillium review seems to have been sparked by one person making mistakes, and agreed the system did work, although perhaps not as quickly as it should.
"I would say the system has worked. It should have been a little bit quicker. It took a little bit long for my point of view, but it did work," said NDP health critic France Gelinas.
Trillium said patients and doctors whose scans are involved in the review will be contacted directly after their tests have been checked.
The external review will be led by a doctor from the Juravinski Cancer Centre in Hamilton and will be made public once it is completed.
"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 higher quality of care at our hospital," said Michelle DiEmanuele, CEO of Trillium Health Partners, in a release.
The radiologist who conducted the scans under review is no longer working with the hospital, Trillium said.
This isn't the first time there were mistakes in cancer tests and other diagnostic procedures in 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.