Why Finding More Cancer With MRI Isn’t Necessarily Better

by Karuna Jaggar, Executive Director

A Danish study published in November 2019 got widespread media coverage for showing that supplemental magnetic resonance imaging (MRI), using a strong magnetic field and radio waves to create detailed images of the organs and tissues, finds more cancerous tumors in women with dense breasts than mammograms alone. But few of the articles published about the first large randomized controlled trial of supplemental MRIs in women with dense breasts, reported that there was no evidence yet that supplementing mammograms with MRIs reduces breast cancer deaths.

Another Danish paper (GS4-07) presented at SABCS looked at the cost effectiveness of MRI in the first randomized trial comparing MRI breast cancer screening with mammography in high risk women, due to family history. There were 1355 women, ages 30-60 with an estimated 20% lifetime risk, enrolled in the study. MRI has been used in the US for routine surveillance for very high risk women with BRCA mutations but half of the women in this study didn’t have a BRCA or p53 mutation.

MRIs picked up more abnormalities than mammograms: the MRIs not only found more invasive cancers, they found more DCIS and more irregularities that turned out not to be cancer (false positives). MRI detected more cancer at earlier stages, but also more false positives. MRI performed best for young women (under 50) with dense breasts and the procedure detected cancer at an earlier stage.

The researcher argued that because average life expectancy for women in the trial was another 34 years, MRI could be an important screening tool. But when questioned, she also acknowledged that overdiagnosis was common and estimated between 17-19% of cancers found were overdiagnosed. The “cost” of finding cancers that don’t matter and don’t need treatment was not factored into her calculation when she concluded that “MRI protocols for familial risk are cost effective.”

It’s important to remember that overdiagnosis (finding and treating cancer that won’t ever become life threatening) is distinct from false positives (finding something suspicious that turns out not to be cancer). Where false positives result in additional testing to rule out cancer, overdiagnosis results in unnecessary treatment, meaning women who are overtreated experience the harms of treatment without any of the benefit.

Which is why I went to the mic to ask about survival data. Because without that, there’s no evidence yet that finding 4 more cancers per 100 (10 v 6) with MRI saves lives. And the high rate of overdiagnosis means that tens of thousands of women will live the remainder of their lives—estimated to be 34 years—with all of the lasting physical, financial, sexual, and emotional toll of overtreatment. Three and a half decades is a long time to endure the life-long—and life changing—effects of unnecessary breast cancer treatment.

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