MRI's High False-Positive Rate Has Little Impact on Choice of Preventive Mastectomy

Magnetic resonance imaging (MRI) falsely detects breast cancer in five out of every six positive scans according to a new study that examined the use of MRI for women with a high, inherited risk of developing the disease. However, this high rate of false-positives does not have a major impact on a woman's decision whether or not to have a prophylactic mastectomy.

The study, published in the April 2008 issue of the journal Annals of Oncology, also demonstrated that MRI was very good at detecting authentic cases of invasive tumors and ductal carcinoma in situ (DCIS), a localized pre-cancer that can develop into invasive breast cancer, although the investigators reported that improvements in detection were still needed.

The researchers, from the Hereditary Cancer Clinic at Radboud University Nijmegen Medical Center (Nijmegen, The Netherlands), tracked 196 women with a BRCA1 or BRCA2 mutation, aged between 21 and 68 for an average of two years (a range of between one and nine years). Once the women had discovered that they carried a BRCA mutation (which gives a lifetime risk of breast cancer of up to 85%, and for ovarian cancer of up to 60%), they made six-monthly visits to hospital to be evaluated by an experienced medical specialist and to have a mammography, and whenever possible, an MRI scan. During their first surveillance visit, the researchers recorded whether the women preferred ongoing surveillance, or prophylactic mastectomy and/or a prophylactic salpingo-oophorectomy (removal of fallopian tubes and ovaries).

During the period of the study, which ran from 1999 to 2005, 41% (81 of 196) of the women had at least one positive MRI or mammogram; breast cancer was detected in 17 women (11 from scanning, four during prophylactic mastectomy and two during the interval between surveillance visits). The researchers found that the sensitivity of mammography (the proportion of true positives) was 41%, for MRI it was 60% and for the two combined it was 71%. The specificity of the techniques (proportion of true negatives) was 93% for mammography and 90% for MRI or a combination of the two.

The lead author of the study, Dr. Nicoline Hoogerbrugge, associate professor and head of the Hereditary Cancer Clinic, said, "When we looked at the positive MRI results, we found that 83% of them could not be confirmed histologically and were, therefore, false-positives: five out of every six positive MRI scans.”

The researchers also found that approximately 6% of the BRCA mutation carriers who had normal findings from their clinical surveillance, mammograms, and MRI scans, and who underwent an intended prophylactic mastectomy, had an unsuspected malignancy. One was only four mm in size and it is known that MRI and mammograms have difficulty detecting lesions this small, but three were DCIS between six and 15 mm. "This indicates that further improvement of early breast cancer detection is still necessary,” said Dr. Hoogerbrugge.

When interviewed during their first surveillance visit, 58 women (30%) expressed a preference for a prophylactic mastectomy. Three had no preference, and the rest preferred to have ongoing surveillance. After some of these women had a positive scan, mastectomy was carried out in 90% of those women who had expressed a preference for it, and in only 31% of those who had preferred surveillance.


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Radboud University Nijmegen Medical Center