The 854 women in the study had been referred for biopsy after an abnormal mammogram.
Researchers used the mammograms to assess global bilateral asymmetry, which was the overall absolute difference in percent fibroglandular tissue volume (%FGV) between the ipsilateral (biopsied) breast and the contralateral (unaffected) breast.
The researchers also assessed local bilateral asymmetry, which was the perilesional %FGV difference in an area twice the size of, but excluding, the biopsy target, and the corresponding area in the unaffected breast.
The women were then divided into quartiles based on breast density asymmetry.
Most of the women had benign breast disease, including proliferative (43%) and nonproliferative (33%) disease, but 23% had carcinoma in situ or invasive breast cancer.
The trend for higher risk of in situ or invasive cancer with increasing breast density asymmetry was observed only in the local analysis. The odds ratio was 1.59 (95% confidence interval, 0.94-2.69) for women in the highest quartile of breast density asymmetry (absolute difference, > 8.23) versus those in the lowest quartile (absolute difference, ≤ –5.55; P = .067).
When compared with women who had proliferative benign disease, women with carcinoma in situ or invasive breast cancer “were more likely to be in the higher than lower quartiles,” said lead investigator Maeve Mullooly, PhD, a research fellow at the Royal College of Surgeons in Dublin.
There was no association between breast density asymmetry and traditional breast cancer risk factors such as age, body mass index, race, and hormone therapy. However, among women diagnosed with benign nonproliferative disease, women with a breast cancer family history were more likely to have higher overall breast density asymmetry.
Study rationale and details
Higher breast density is a known risk factor for breast cancer. Breast asymmetry also has been reported as a possible risk factor (Breast Cancer Res. 2006;8[2]:R14), and incorporation of breast density asymmetry into traditional risk factors in one study improved risk prediction (Breast Cancer Res. 2017 Mar 14;19[1]:29).
Building on that work, the goal of Dr. Mullooly’s study was to “learn how to better use breast density to inform breast cancer risk prediction,” she said.
To that end, her team turned to 854 women enrolled from 2007-2010 in the National Cancer Institute’s Breast Radiology Evaluation and Study of Tissues Project, a cross-sectional molecular epidemiologic study designed to understand how breast density measures are related to breast cancer etiology.
Most of the women were non-Hispanic white. The mean age was 51 years (range, 40-65), and the median body mass index was 25 kg/m2.
About three-quarters of the women (76%) had a breast density asymmetry of at least 2% on the global analysis, with 43% having higher %FGV in the biopsied breast and 33% having higher %FGV in the unaffected breast. In all, 89% of women had local breast density asymmetry, with higher density in the biopsied breast in 61% of women and higher density in the contralateral breast in 28%.