A "benign" lump found in the breast of about 100,000 women per year is considered harmless and thus often ignored but findings of a new study suggest that the mass, known as atypical hyperplasia is associated with more breast cancer risks than previously believed.

Lynn Hartmann, from the Mayo Clinic in Rochester, and colleagues found evidence that women who have atypical hyperplasia have increased likelihood of developing breast cancer compared to what experts believed.

For their study published in the New England Journal of Medicine on Jan. 1, Hartmann and colleagues followed almost 700 women who were diagnosed with atypical hyperplasia. Of these, 143 developed breast cancer, a disease that affects 12 percent of women in the U.S.

The researchers also looked at another group of women who have the same condition and found that in both instances, the data indicate that women who were diagnosed with atypical hyperplasia have 7 percent increased odds of developing breast cancer five years after their biopsy, 13 percent likelihood after a decade and 30 percent after a period of 25 years.

Hartmann said that experts have long known that atypical hyperplasia is associated with increased odds for breast cancer but their study sheds light on the extent of this risk. The researcher said that the cancer risks of women in this group can be curbed with regular screening through mammograms and MRIs and use of anti-estrogen therapies.

"Clinicians should understand that atypical hyperplasia confers an absolute risk of later breast cancer of 30% at 25 years of follow-up," the researchers wrote. "Guidelines for high-risk women should be updated to include women with atypical hyperplasia; screening MRI should be considered an option for them, to be performed in addition to mammography."

The researchers likewise noted that many of the participants in their study developed an estrogen receptor-positive cancer that needs estrogen to grow, which means that chemopreventive medicines such as tamoxifen may help lower their risks.

Although some women may be apprehensive in taking medication for fear of side effects, Hartmann said that in breast cancer prevention trials that involved participants with atypical hyperplasia, using hormonal therapies reduced the risks of breast cancer by at least 50 percent.

"We hope that the combination of information provided in this report on actual risks of breast cancer and actual risks of side effects will help patients and practitioners make informed decisions on the best treatment approaches for women with atypical hyperplasia," Hartmann said.