A review of radiology scans at University Hospital Kerry has found that 11 patients had their cancer diagnoses delayed, and four of these patients have since died.
The review was of all radiology images reported by a single consultant radiologist at the hospital between March 2016 to July 2017.
It was prompted by three "serious reportable events" in July and August 2017.
The look-back review said 11 patients had their diagnosis delayed, which had a "serious impact on their health".
This included the three initial cases which prompted the review.
It also found that three patients had undiagnosed cancer, which had not previously been identified.
In total 46,234 images - including CT scans, ultrasounds and chest x-rays - were examined.
University Hospital Kerry | Image: hselibrary.ie
It's made a number of recommendations - including an external review of the management of the radiology department, a review and enhancement of incident reporting and a definition of acceptable volumes of work for individual radiologists.
In a statement, the South/South West Hospital Group apologised "sincerely and unreservedly" to all patients and families.
Patients who have any concerns following the publication of the look-back review can contact a helpline on 1800-742-900.
This will operate between 9.00am to 9.00pm on Wednesday and 9.00am to 5.00pm from Thursday onwards.