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Women with a false positive mammogram or breast biopsy result have an increased risk of breast cancer over the next 10 years compared with those with a true negative result, a new report has found.

The study, in the December issue of Cancer Epidemiology, Biomarkers & Prevention, used data on more than two million mammograms nationwide, from women age 40 to 74, examined from 1994 to 2009. Compared with women who had a true negative result, a woman with a false positive referred for further imaging had a 39 percent increased relative risk for cancer. Among those with a false-positive result who were referred for biopsy, the relative risk was 76 percent greater.

The difference persisted for 10 years, and women with extremely dense breasts who had a false-positive result with a biopsy were at the highest risk.

Still, the absolute increase in risk remained low, around 1 percent, said the lead author, Louise M. Henderson, an assistant professor of radiology at the University of North Carolina.

“While any increase in risk is worrisome,” she added, “we don’t want women to be overly concerned, because the absolute risk is small. Instead, we want women to think about this as another risk factor, and talk about it with their physicians.”

Women with false-positive results were no more likely to have additional screenings, so the difference could not be attributed to diagnostic bias — that is, to finding more cancers because there were more examinations.

The mechanism is unclear, but the authors suggest that a radiologist or pathologist may see, and send for further review, an abnormality that is itself benign, but is associated with a future cancer.