The errors, which happened at the Kent and Canterbury Hospital, were revealed in a review of all cancer diagnosis services at the hospital. It exposed one "rogue" histopathologist who has been ordered to have "remedial training".
The hospital is now checking all the breast biopsies - samples of tissue removed for diagnosis - that he examined between 1990 and 1997. It is likely that more were given inappropriate treatment. But the review by the Royal College of Pathologists concluded that the overall error rate was within acceptable standards.
Experts from the college looked at 2,621 patient histories and identified seven cases where "diagnostic discrepancies" had "a definite influence on clinical outcome". A further 35 were defined as "misrepresentation" or "oversights"which could have affected the way people were treated.
David Astley, chief executive of the East Kent NHS Trust, said: "So far it is, sadly, clear that two patients had unnecessary mastectomies. We have seen one of them and told her and her family what has happened. The other is abroad."
Apologising for the errors, he added: "There can never be any absolute guarantees in life, but I would say the lessons have now been learnt, not just by the hospital but by the NHS as a whole. We are confident we have now got multiple checking procedures in place and an open culture in which mistakes can be recognised".
The review of the pathologist's work would involve "a few hundred women" and take "months rather than years" to complete. "We are likely to find only a very small number who have been inappropriately treated and we will announce the findings once the work is complete," Mr Astley said.
The Kent and Canterbury Hospital laboratory in Canterbury has now been closed.Reuse content