The review covers work done between April 1, 2012, and March 31, 2013, at the Mississauga Hospital and the Queensway Health Centre in Toronto.
All of the patients involved have all been notified by letter. A total of 189 people had multiple scans, a spokeswoman for the hospital said.
"There is one patient that has been notified that had a clinically significant event," said Dr. Dante Morra, chief of medical staff at Trillium Health Partners, who spoke to the patient.
"I think it was difficult for that patient."
The radiologist is Dr. Ivo Slezic.
The spokeswoman said the radiologist had worked at the Trillium hospitals for 33 years and his "privileges" to provide diagnostic imaging were restricted as of April 1, 2013. The hospital notified the province's College of Physicians and Surgeons about the restriction on June 6, according to the college's online physician database.
Trillium Health has launched an external review team, led by Dr. Brian Yemen, chief of diagnostic imaging, Juravinski Hospital and Cancer Centre and McMaster University Medical Centre at Hamilton Health Sciences.
Yemen said he appreciates how difficult the news is for patients and families.
"We are doing everything we can to complete the review quickly and properly," Yemen said in an interview. "Really my job is to make sure the patients have received the right care, and if they haven't, then to identify and take the proper steps immediately."
Yemen said today that the review will be done in about six weeks. As many as 17 radiologists on the review team will start with the most recent cases where there's probably the biggest potential impact on patient care, he said.
They will base their work on a U.S. radiology college's acceptable error rate of about two per cent, because an error rate higher than that raises flags, Yemen said.
Yemen said the scope of the review could expand if needed.
"Ontarians quite rightly expect the highest standard of care from their hospitals, and from the physicians and health professionals who work in them," Health Minister Deb Matthews said in a statement.
"Our hospitals take concerns about quality of care very seriously. I have been assured that affected patients are being properly informed and that their cases are being reviewed as quickly as possible so that followup care can be provided."
The head of the hospital issued an apology and said the hospital will follow up directly with all patients as soon as their tests have been reviewed.
"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," said Michelle DiEmanuele, chief executive of Trillium Health.
Trillium Health is made up of the Credit Valley Hospital, Mississauga Hospital, and Queensway Health Centre, and serves a population of 1.15 million people.