Drop in Breast Density Key to Tamoxifen Lowering Risk

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Tamoxifen is a hormonal therapy medicine that can be taken by women with a much higher-than-average risk of breast cancer to reduce risk. A study found that tamoxifen was successful at reducing risk only in high-risk women whose breast density significantly decreased during the first year or so of taking tamoxifen. The results were presented at the 2008 San Antonio Breast Cancer Symposium.

More than 7,000 women at high risk for breast cancer, but who had never been diagnosed, participated in the study, called IBIS-1. About half the women took tamoxifen for 5 years. The other half of the women took a placebo (sugar pill) for 5 years. After about 8 years of follow-up, the women who took tamoxifen were 27% less likely to develop breast cancer compared to the women who took the placebo.

During the study, the researchers measured the breast density of the women who took tamoxifen. Breast density is the proportion of fatty tissue compared to non-fatty tissue in the breast. A denser breast has more non-fatty tissue in it than a less-dense breast. Breasts usually get a little less dense over time.

More than 45% of the women who took tamoxifen had their breast density drop by 10% or more during the first 12 to 18 months of taking tamoxifen. The other 54% of these women had their breast density drop by less than 10% during the same time period. The researchers compared the breast cancer risk of the two groups of women who took tamoxifen to women who took the placebo.

The results:

  • The women who took tamoxifen who had a 10% or more drop in breast density within 12 to 18 months of starting tamoxifen were 63% less likely to develop breast cancer compared to the women who took the placebo.
  • The women who took tamoxifen who had less than a 10% drop in breast density within 12 to 18 months of starting tamoxifen had the same breast cancer risk as the women who took the placebo.

This means that women who took tamoxifen and didn't have a 10% or greater drop in breast density within the first year or so on tamoxifen had no reduction in breast cancer risk. Essentially, the tamoxifen wasn't effective for these women.

Tamoxifen may cause bothersome and sometimes serious side effects, including hot flashes and life-threatening blood clots. Many women who might benefit from taking tamoxifen to lower breast cancer risk choose not to take tamoxifen because they're worried about these side effects, as well as whether tamoxifen actually will reduce breast cancer risk.

In a perfect world, tamoxifen would only be given to high-risk women whose breast cancer risk would be reduced significantly. This study may offer a way to figure out if a woman at high risk for breast cancer is getting protective benefits from taking tamoxifen. The study suggests that only women whose breast density drops by 10% or more after 12 to 18 months of taking tamoxifen will benefit from 5 years of tamoxifen. Still, these results are EARLY results. More research is needed before lower breast density can be used to measure the effectiveness of tamoxifen.

If you have a higher-than-average risk of breast cancer and you're taking (or thinking about taking) tamoxifen, you might want to ask your doctor about this study. Together you can decide if and for how long tamoxifen treatment makes sense for you.

Visit the Breastcancer.org Lower Your Risk section to learn more about ways to lower breast cancer risk if your risk is considered to be higher than average.

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