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Police have been called over the blunder amid fears someone has found the document and plans to make it public.
Both agencies involved - the Southern District Health Board and Pacific Radiology - say that finding the file is of the "highest priority".
The print-out, which contained screening information, diagnoses, and treatment plans for about 26 women - plus their names, ages, ethnicities and hometowns - was mislaid in central Dunedin on Wednesday.
The Herald on Sunday understands the doctor, a Pacific Radiology employee, lost it somewhere on the 1.5km journey between the breast cancer screening service's Bond St clinic and Dunedin Hospital.
He was going to use it to discuss the patients in a clinical meeting at the hospital.
Despite staff retracing the doctor's steps, the file has not yet been recovered - prompting the health board to launch a full investigation and apologise individually to each patient.
It was also reported to Privacy Commissioner John Edwards, who said he had not yet looked into the matter but that he would expect the agency to be contacting those affected straight away.
"Some women might not have concerns but for others it might be very distressing," Edwards said. "I'm sure [health authorities] are mortified but, again, it's not something that can be taken lightly."
Edwards said he would have expected a high level of oversight and control over that kind of information. At least one other health board had shifted to using lockable iPads rather than paper files after a similar incident, he said.
Southern DHB chief executive Chris Fleming said it had contacted all the patients on Friday, and would send each a letter with more information about the privacy breach.
"It's just a human error but it's really regrettable," Fleming said. He said the doctor who lost the file would feel terrible, as did all the staff involved.
Fleming said while some of the patients understood it was a mistake, others were extremely concerned, which was understandable.
"Once again we apologise for the breach and understand how distressing that can be."
In its letter, the Southern District Health Board and Pacific Radiology would promise to investigate the error, and work to ensure it doesn't happen again.
Fleming said details of what an investigation would entail were not yet certain, as the health board had been focusing on addressing the issue in the event the information did make it into the public arena.
Fortunately, all of the patients had already been told their diagnoses - not all of them had breast cancer as some of the screens were clear - but the information was still highly personal.
Fleming said while the doctor who lost the file was not a Southern District Health Board employee, he took responsibility for the error, as Pacific Radiology was contracted to carrying out breast cancer screening for the board.
He said it highlighted why the board wanted to move to a paperless system, which it was working towards.