New research indicates that when the use of tamoxifen (Nolvadex®) results in a reduction of breast density, this change in breast density serves as an indicator that the drug is effectively working to reduce the risk of breast cancer. In other words, changes in breast density may indicate whether a woman is actually benefiting from the use of tamoxifen. The results of this study were presented at the San Antonio Breast Cancer Symposium on Saturday December 13, 2008.
Tamoxifen is an anti-estrogen that has been shown to reduce the risk of breast cancer among women at high risk of the disease. It has also long been the mainstay of hormone therapy for breast cancer. Several randomized studies have shown that treatment with tamoxifen for five or more years significantly reduces the risk of breast cancer recurrence.
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 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.
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.
 Cuzick J, Warwick J, Pinney L, et al. Change in breast density as a biomarker of breast cancer risk reduction; results from IBIS-1. 31st Annual San Antonio Breast Cancer Symposium. December 10-14, 2008. Abstract 61.
 Fisher B, Costantino JP, Wickerham DL et al. Tamoxifen for the Prevention of Breast Cancer: Current Status of the National Surgical Adjuvant Breast and Bowel Project P-1 Study. Journal of the National Cancer Institute. 2005;97:1652-62.