Magnetic resonance imaging (MRI) falsely detects breast cancer in five out every six positive scans, research shows, however such a high rate of false positives does not impact upon a woman's decision to have prophylactic mastectomy.

Researchers at the Hereditary Cancer Clinic at Radbound University Nijmegen Medical Centre analysed 196 women aged between 21 and 68 with a BRCA1 or BRCA2 mutation over two years.

Females with a BRCA mutation made six-monthly visits to hospital for a mammography and an MRI, where researchers recorded whether women had a preference for ongoing surveillance, or prophylactic mastectomy and/or a prophylactic salpingo-oophorectomy on their first visit.

Analysis showed 41 per cent of the women had at least one positive MRI or mammogram, with breast cancer detected in 17 women (11 from scanning, four during prophylactic mastectomy and two during the interval between surveillance visits).

The sensitivity of mammography (the proportion of true positives) was 41 per cent, while for MRI it was 60 per cent. For the two combined it was 71 per cent.

The specificity of the techniques (proportion of true negatives) was found to be 93 per cent for mammography and 90 per cent for MRI or a combination.

"When we looked at the positive MRI results, we found that 83 per cent of them could not be confirmed histologically and were, therefore, false positives: five out of every six positive MRI scans," Dr Nicoline Hoogerbrugge, head of the Hereditary Cancer Clinic, said.

In prophylactic mastectomy 30 per cent of participants expressed a preference for the treatment, three had no preference, while the remainder preferred to have ongoing surveillance.

After some of these women had a positive scan, mastectomy was carried out in 90 per cent of those women who had expressed a preference, and in only 31 per cent of those who had preferred surveillance.

Researchers suggest that personal beliefs and the psychological impact of women's family medical histories appeared of great import in women's decision-making process.

Approximately six per cent of BRCA mutation carriers showing normal findings from their clinical surveillance, mammograms and MRIs, and who underwent an intended prophylactic mastectomy, had an unsuspected malignancy.

One of these tumours was four millimetres in size, however three were DCIS between six and 15mm, causing researchers to conclude that further improvement of early breast cancer detection was necessary.

The findings are published in the Annals of Oncology.

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