Reduced Breast Density May Indicate a Response to Tamoxifen

The International Breast Intervention Study I (IBIS-I) examined the effects of tamoxifen in the prevention of breast cancer and involved over 7,000 participants. During the study, women underwent a baseline mammogram, as well as mammograms at 18, 36, and 54 months to monitor for the development of breast cancer. When researchers observed a correlation between breast density reduction and reduced breast cancer risk, they conducted an analysis of a subpopulation of the IBIS-I participants.

Breast density refers to the extent of glandular and connective tissue in the breast. Breasts with more glandular and connective tissue—and less fat—are denser. Women with higher breast density are at increased risk of developing breast cancer.

The subpopulation included 120 women who developed breast cancer and 943 “controls” who did not develop breast cancer. The researchers examined the baseline mammograms, as well as the mammograms that took place 12-18 months after treatment with tamoxifen was initiated. The results indicated that 46% of the women in the tamoxifen group experienced a reduction in breast density of 10% or more. Furthermore, the women whose breast density was reduced by 10% or more experienced a 52% reduction in the risk of breast cancer relative to the control group. Conversely, women whose breast density was not reduced by 10% had a non-significant 8% reduction in breast cancer risk.

The researchers concluded that the changes in breast density induced after 12-18 months of treatment with tamoxifen can serve as an indicator, or biomarker, of the impact of tamoxifen on the reduction of breast cancer risk. They assert that changes in breast density may serve as an early indicator of treatment efficacy. Furthermore, these changes in density could even be used to determine which women are benefiting from treatment with tamoxifen and those who might need a different risk reduction approach.