Women with atypical hyperplasia of the breast have a 1 percent increased risk of developing breast cancer each year, suggests a study published Wednesday in the New England Journal of Medicine.

Previous research has shown that females with atypia, a precancerous condition in which breast cells begin to grow out of control and cluster in abnormal patterns, have a four- to five-fold increased relative risk. But few studies have assessed patients’ absolute risk, or the chance that she will develop the breast cancer over a period of time, according to a news release.

To define this risk, a team of researchers at the Mayo Clinic followed 698 women with atypia who had been biopsied between 1987 and 2001. They used medical and pathology records, as well as follow-up surveys, to determine which study participants developed the disease and when.  After an average of 12 and a half years, 143 of the women developed the disease.

Five years after a biopsy, 7 percent of these women developed the disease, and after 10 years the number jumped to 13 percent. Twenty-five years later, 30 percent of these women developed breast cancer.

To validate their results, researchers compared these findings with biopsies from a separate group of women with atypia, data that researchers at Vanderbilt University gathered.

Studying a patient’s pathology specimen also revealed that as the extent of atypia— measured by the number of separate foci, or atypia lesions— in a biopsy increased, so did breast cancer risk, according to the news release. Twenty-five years post-biopsy, 47 percent of women with three or more foci of atypia in the biopsy had developed breast cancer, compared to 24 percent of women with one focus.

Study author Amy Degnim, a breast cancer surgeon at Mayo Clinic, said the actual data used in the study is preferable to a statistical model, which is hypothetical.

The researchers say the findings suggest a need for greater screening and specialized medicine for the more than 100,000 women diagnosed annually with this condition. Atypical hyperplasia is thought to be a low risk factor for breast cancer.

"By providing better risk prediction for this group, we can tailor a woman's clinical care to her individual level of risk," lead study author Lynn Hartmann, an oncologist at Mayo Clinic, said in the news release. "We need to do more for this population of women who are at higher risk, such as providing the option of MRI screenings in addition to mammograms and encouraging consideration of anti-estrogen therapies that could reduce their risk of developing cancer."

Clinical trials have shown that anti-estrogen medications such as tamoxifen can lower the risk of breast cancer by 50 percent or more. However, Degnim said that many women with atypia are not taking their medications because their physicians haven’t had solid estimates of their breast cancer risk and administer treatment accordingly.