A review into the work of a locum consultant radiologist has so far identified "major discrepancies" affecting 12 cases.
A full lookback review of 13,030 radiology images was launched last month.
The doctor worked at hospitals run by the Northern Health Trust between July 2019 and February 2020.
The review steering group chair said it was "images in levels one and two that we are most concerned about".
"To date there are 12 level ones and twos [approximately 0.5% of the total number reviewed]," said Dr Seamus O'Reilly, the Northern Trust medical director.
"Most of these concern CT scans where inaccurate initial reading of the scans could, or is likely to, have had an impact on the patient's clinical treatment and outcome."
More than 9,000 patients have been contacted as part of the review, which is looking at radiology images taken in Antrim Area, Causeway, Whiteabbey and Mid Ulster Hospitals as well as the Ballymena Health and Care Centre.
Source: BBC News, 28 July 2021