MRIs of dense breasts find more cancers, but also false alarms

Giving women with very dense breasts an MRI scan in addition to a mammogram led to fewer missed cancers, but also to a lot of false alarms and treatments that might not have been needed, a study has found.

The results gave a clearer picture of the tradeoffs involved in such testing, but they did not answer whether it saves lives.

For women with dense breasts trying to decide on screening, “the dilemma remains,” Dr. Dan Longo of the New England Journal of Medicine wrote in an editorial published with the study this week.

About half of women over 40 have dense breasts and about 10 per cent have very dense ones. That raises their risk of developing cancer and makes it harder to spot the disease on mammograms.

U.S. regulators are making rules to require that women are given breast-density information when they have mammograms. But what to do if you have dense breasts is unclear — it’s not known if more or different types of screening, such as MRIs or ultrasounds, help.

The study involved more than 40,000 Dutch women, ages 50 to 75, with very dense breasts who had normal results from a mammogram, a screening X-ray offered every two years in the Netherlands.

About 8,000 were also offered an MRI, which uses powerful magnets to create detailed images, and 4,783 women agreed.

Researchers tracked how many breast cancers were detected in each group within two years. Finding more “interval cancers” implies that the initial screening might have missed them. The rate of these cancers after two years was twice as high in the group that was only offered mammograms.

This suggests that adding MRIs to initial screening caught more cancers, but they also gave a lot of false alarms — about 80 per 1,000 scans.

Three quarters of women who had a biopsy after a questionable MRI turned out not to have cancer.