Scans given to women with a high risk of hereditary breast cancer falsely detect the disease in five out of six cases, research has shown.
However, the study found that ambiguous test results had only a limited effect on whether or not patients chose to have breasts removed as a preventative treatment.
Women with BRCA1 or BRCA2 gene mutations have an 85 per cent chance of developing breast cancer during their lifetime.
They qualify for intensive screening using both conventional mammography X-rays and magnetic resonance imaging (MRI) scans.
New research by scientists in the Netherlands found that although MRI scans were good at picking up genuine cancers, they also produced a very high proportion of false positive results.
The scientists studied 196 women aged 21 to 68 with BRCA1 and BRCA2 mutations for two years.
During the study 81 of the women had at least one positive MRI scan or mammogram result. The probability of a false indication of cancer from an MRI scan was 83 per cent.
Breast cancer was detected in a total of 17 women. MRI scans spotted 60 per cent of the tumours, and mammography scans 41 per cent.
The findings were published today in the cancer journal Annals of Oncology.
Dr Sarah Rawlings, the head of policy at the charity Breakthrough Breast Cancer, said: "MRI, together with mammography, is the most effective form of screening for this group of women. 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."
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