Women with dense breasts have a sharply higher risk of breast cancer compared to those with less dense breasts, which have a higher percentage of fat. Dense breasts are associated with increased risk of both hormone receptor positive (ER+/PR+) and mixed hormone receptor (ER+/PR- and ER-/PR+) breast cancer. Breast cancer survivors with dense breasts have a higher rate of new primary breast cancer (i.e., a new tumor that arises after treatment). However, women with a reduction in breast density over time have a lower risk of breast cancer than women whose breast density remains stable. Now a new study has reported that changes in breast density can determine whether endocrine treatment is working.

Breast cancer treatment can reduce breast density

Breast density is not fixed; it can be increased or reduced by environmental exposures and diet. chemotherapy reduces breast density, especially among younger women. Radiotherapy significantly reduces the risk of local recurrence among early-stage breast cancer patients with dense breasts. Aromatase inhibitor treatment has also been shown to reduce breast density.

Successful endocrine treatment is linked to reduction in breast density

The study referenced at the beginning of this news article was designed to determine whether a short-term change in breast density is a valid biomarker for response to adjuvant endocrine therapy. Adjuvant endocrine therapy consists of anti-estrogen treatment with aromatase inhibitors (Arimidex, Femara or Aromasin) and/or tamoxifen that takes place after breast cancer surgery and chemotherapy and/or radiation treatment. To conduct the study, the authors analyzed data concerning 1,065 estrogen receptor positive (ER+) breast cancer patients. The women underwent surgery during the period 2003 to 2006, which was followed by at least two years of endocrine treatment. Percent mammographic breast density reduction was defined as the absolute difference in breast density of mammograms taken before surgery and eight to 20 months after the beginning of endocrine treatment. The women were followed for a median of 68.8 months.

Changes in breast density were then compared to the outcomes of the breast cancer patients. Breast cancer recurred in 7.5% (80/1065) of the overall study group during follow up. The average reduction in breast density was 5.9% (range: -17.2 to 36.9%). Larger breast density reductions were found among women who were under 50, had high initial breast density, and/or experienced a long interval between the beginning of endocrine therapy and the follow-up mammogram. Large tumor size, positive lymph nodes, high proliferation index (defined as Ki-67 ≥ 10%), and low reduction in breast density each were found to be independent factors predicting greater likelihood of recurrence.

Compared with the group showing the greatest breast density reduction (reductions ≥ 10%), the risk of recurrence was 2.26 times higher for women who did not experience any reduction in breast density. The authors conclude that change in breast density during approximately the first year of adjuvant endocrine therapy is a significant predictor of five-year recurrence in women with ER+ breast cancer.

Please see our articles on aromatase inhibitors, tamoxifen and breast density for more information.