An internal audit into the Southern District Health Board's breast screening programme had identified possible delays in diagnosing breast cancer in 28 women over a three-year period, Ministry of Health chief medical officer Dr Don Mackie said.
The problem relates to the reading of mammograms at BreastScreen HealthCare, the service for Otago and Southland women.
An urgent investigation is under way, including a review by external senior radiologists.
"We fully expect all the questions women will ask will be answered by the investigation as we get that put in place. Unfortunately, we don't know the answers just yet," Dr Mackie said in a statement.
The service screens about 16,000 women a year.
A routine three-year audit under way was also raising concerns, which would be addressed in the investigation, Dr Mackie said.
"Additional checks are being set up at key steps in the screening and assessment pathway."
In a memo to staff yesterday, new SDHB chief executive Carole Heatly apologised to those who learnt of the problem from the media.
"The executive team had planned to discuss this issue with affected staff and to inform you all of the investigation before it was placed in the media.
"Regrettably, this was not possible [as] the first media inquiries were made after 7pm [on Wednesday]." Ms Heatly said she had met breast screening staff, who were being offered support.
In a later statement, Ms Heatly said the DHB would work closely with the Ministry of Health.
"I would like to reassure the community that, while our services are generally of a very high standard, we are committed to improving our performance in breast screening and will work closely with the Ministry of Health-led review."
University of Otago cancer screening authority Associate Prof Brian Cox was unaware of the specific issue at BreastScreen HealthCare, but said it was the third time the service had been implicated in problems in a decade.
"Top-quality services, I think, have fewer events than this."
Errors in screening were more dangerous than in other areas of the health-care system, because people did not have symptoms.
Reading breast screens could be "extremely boring", because so many were not abnormal, meaning a high level of awareness and attention was required, he said.
It was vital to know if all the mammograms had been double-read by two radiologists, which was considered best practice, Prof Cox said.
The DHB spokesman confirmed that double-reading was standard practice at BreastScreen HealthCare.
An item in a regular newsletter to GPs from the DHB yesterday advised of the departure of breast service radiologist Dr Shelley Boyd, who has left for Christchurch.
Some mammograms were being outsourced to Otago Radiology as a consequence of the vacancy, the newsletter said.
A DHB spokesman advised the Otago Daily Times there was not a connection between Dr Boyd's departure and the issues raised in the internal audit.
When fully staffed, the service has 1.2 full-time-equivalent radiologists. At present, there was a 0.7 full-time-equivalent vacancy, the spokesman said.
Helpline
Women concerned about the issue should call 0800-214-579.
