This MRI image shows a tumour, but there are often false alarms
Lumps detected in women at a high risk of breast cancer using hi-tech MRI scans overwhelmingly turn out to be false alarms, a Dutch study suggests.
But while researchers found five out of six scans which suggested a problem were wrong, they were nonetheless very effective at spotting invasive cancers.
And while false-positives caused anxiety, the study did not find women were rashly opting for mastectomies.
The findings were published in the Annals of Oncology.
Women with certain genes have as much as an 85% risk of breast cancer.
In the UK, NHS guidelines say that a woman with a family history of the disease may be offered yearly MRI scans if her doctors think it is appropriate, in addition to the recommended mammogram.
MRI scans - which use magnetic resonance imaging - are more sensitive than standard mammograms, and as such are seen as particularly effective in picking up early breast cancers in younger women with denser breast tissue.
But this very sensitivity is what makes them more likely to spot abnormalities which may turn out to be completely benign.
Researchers at the Hereditary Cancer Clinic at Nijmegen Medical Centre followed 196 women with the BRCA1 or BRCA2 gene mutation which has been linked to breast cancer.
On their first visit, they were asked whether they had a preference for ongoing surveillance in the form of scans, or whether they would rather have their breasts removed as a precaution.
About 30% said they wanted a protective mastectomy, three expressed no preference and the others wanted regular check-ups.
All also went on to receive a scan. About 83% of the MRI scans which appeared to detect a tumour were later found to be so-called "false positives".
However, the positive scan appeared to do little to sway decisions on breast removal.
Some 90% of the group who had wanted a mastectomy went on to have one, while fewer than a third of those who received a positive result and had originally opted for surveillance opted for breast removal.
"The final decision to actually undergo prophylactic mastectomy appeared to be determined more frequently by a woman's prior preference than by a positive scan," said Dr Nicoline Hoogerbrugge, who led the research.
There was little information available about how women made the choice, she added. "Both genetic counselling and breast cancer surveillance are events in the entire decision-making process, but appear to have a limited impact."
But the researchers did confirm that the MRI scan was more effective in picking up genuine tumours and pre-cancerous developments.
The proportion of true-positives for mammography was 41%, while for MRI it was 60%.
Breakthrough Breast Cancer said the findings should not curtail access to MRI scans for those women at high risk.
"MRI, together with mammography, is the most effective form of screening for this group of women although this method can result in false positives," said head of policy Dr Sarah Rawlings.
"It is vital that all those eligible have access to this service and also receive clear and accurate information about the risks and benefits of MRI so they can make an informed decision."
Breast Cancer Care clinical director Dr Emma Pennery said: "While MRIs can result in false positive findings, this is far outweighed by their ability to detect tumours earlier in their development compared to mammography in high risk groups."