Breast MRI Scan False-Positive Rate Drops on Repetition

Women who get breast MRI scans may have higher false-positives on their baseline studies, but those rates drop on subsequent scans, according to a study to be published in the April issue of Radiology.

Study co-author Martha B. Mainiero, MD, notes that while MRI is an excellent screening tool for breast cancer, the higher rate of false-positive results keep some women from undergoing the exam.

“The goal of our study was to determine if the availability of prior MR images for comparison reduces the rate of false-positives associated with the initial MRI breast screening exam,” said Mainiero, director of the Anne C. Pappas Center for Breast Imaging at Rhode Island Hospital and associate professor of diagnostic imaging at The Warren Alpert Medical School of Brown University in Providence, R.I.

Breast mammography has a known pattern of higher false-positive rates on first screenings than subsequent studies. Authors Dr. Mainiero and Gil Abramovici, M.D. wanted to look for a similar pattern with breast MRI scans. And they found it.

Breast MRI exams are often more effective than mammography at finding suspicious areas in the breast tissue, however these findings are less specific - they can’t always differentiate between cancer and benign lesions, resulting in the need for additional testing.

The Radiology study focused on 650 MRI studies with contrast material from high-risk patients. Almost half (307) were baseline studies, and the remainder (343) were yearly or repeat scans with previous comparison studies available for review. The screenings were done at Rhode Island Hospital, during a 15-month period between 2007 and 2008. The study notes that the patients undergoing the baseline studies were different patients than those undergoing the yearly or repeat scans, though they had similar indications or family/personal history.

The study found that women getting the baseline MRI scan were almost four times more likely (10.1 percent) to get a six-month follow-up recommendation for suspicious findings than patients who had at least one previous MRI scan with a comparison available (2.6 percent).

For the baseline group, the false-positive rate was 13 percent, compared with 5.6 percent in the other group.

As for cancer detection, two women in the baseline MRI group were found to have cancer (0.65 percent), while four women were diagnosed with breast cancer in the repeat group (1.17 percent).

The authors concluded that the drop in the false-positive rate is because benign breast lesions examined as a result of a suspicious baseline study will be considered benign during subsequent scans, as long as the lesions remain stable.
“False positives are a risk of the breast MRI procedure, but the rate decreases following the initial round of screening,” Dr. Mainiero said. “This information should provide reassurance for high-risk patients who are considering undergoing annual MRI screening exams.”