By Megan Rauscher

NEW YORK (Reuters Health) - In women at high risk for developing breast cancer who take tamoxifen to help prevent the disease, a reduction in breast density as seen on mammography is a strong indicator that the drug is working, according to research reported at the 2008 San Antonio Breast Cancer Symposium.

"Our findings suggest that the impact of tamoxifen on risk reduction is predictable by the changes it induces in breast density after 12 to 18 months of treatment," Dr. Jack Cuzick from Wolfson Institute of Preventive Medicine, London, UK and colleagues reported.

If there is a reduction in the density of the breasts, tamoxifen is having an effect; if density is the same, it may not be a beneficial drug for the individual woman, they found.

"Women with dense breasts typically are at four to five times the risk of developing breast cancer than women that don't have dense breasts," Cuzick told the conference. It is very simple to measure breast density, but "at the moment, this information is not used very much," he added.

In the previously reported International Breast Intervention Study I (IBIS-I), the investigators found that tamoxifen reduced the risk of breast cancer by up to 40 percent in women at high risk for the disease. They also observed that tamoxifen reduced breast density as seen on a mammogram.

In the current analysis, they focused on whether the change in breast density after 12 to 18 months of tamoxifen could predict the subsequent impact of tamoxifen on the development of breast cancer.

The investigators analyzed mammograms obtained at the start of the study and after 12 and 18 months of tamoxifen treatment in 120 study participants who developed breast cancer and 943 who did not.

"Compared to all of the women in the placebo arm," noted Cuzick, "if you took the women who received tamoxifen but their breast density was reduced by less than 10 percent, in those women there was no benefit of tamoxifen in terms of prevention. They had the same risk of developing breast cancer as the control population."

Conversely, Cuzick said, "The women who had a 10 percent or more reduction in breast density -- and that was a substantial number, over 40 percent -- they had almost a two-thirds reduction in breast cancer."

By measuring changes in breast density between mammograms in high-risk women receiving tamoxifen, "it may be possible to determine which women are actually benefiting from the intervention (and should therefore continue treatment), and those who might benefit from alternative risk-reducing strategies," Cuzick and colleagues conclude.

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