Research Suggests Link Between Tamoxifen and Risk of Hormone-Receptor-Negative Cancer in Opposite Breast

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Much research has shown that the hormonal therapy medicine tamoxifen reduces the risk of hormone-receptor-positive cancer coming back (recurrence) and improves the overall survival of women diagnosed with hormone-receptor positive disease.

Researchers found that there may be a link between taking tamoxifen for 5 years and a higher risk of developing hormone-receptor-negative cancer in the opposite breast.

Hormone-receptor-negative breast cancer is less common than hormone-receptor-positive breast cancer. About 20% of breast cancers are hormone-receptor-negative. Hormone-receptor-negative breast cancer that also is HER2-negative is called "triple-negative" breast cancer and is considered more aggressive and harder to treat than hormone-receptor-positive breast cancer.

While these results are troubling, the researchers were quick to point out that the findings don't change tamoxifen's risk-benefit ratio. It's also worth noting that the study looked at the medical records of more than 1,000 women in four counties in Washington state. While the study looked at a large number of women, it was geographically limited to only one state. It's possible that something in the environment in the area may have been a factor in the results. More research is needed to confirm the link between taking tamoxifen for 5 years and a higher risk of developing hormone-receptor-negative cancer in the opposite breast.

Some of the cancers in this study were diagnosed almost 20 years ago. At that time, tamoxifen was the standard hormonal therapy medicine; aromatase inhibitors were not commonly used. More research also is needed to see if taking tamoxifen for 2 years and then switching to an aromatase inhibitor for 3 years has the same link to a higher risk of hormone-receptor-negative cancer in the opposite breast. The study reviewed here suggests that taking tamoxifen for less than 5 years may not be associated with the same increase in risk of hormone-receptor-negative cancer.

This study only looked at women that had been diagnosed with breast cancer. It's not clear if the possible link between taking tamoxifen for 5 years and a higher risk of hormone-receptor-negative breast cancer would apply to women at higher-than-average risk but who have never been diagnosed who are taking tamoxifen to reduce risk.

Research continues to show that an aromatase inhibitor is the best hormonal therapy medicine to start with after initial breast cancer treatment for postmenopausal women diagnosed with early-stage, hormone-receptor-positive breast cancer. But tamoxifen is still a good choice, depending on your unique situation. For a number of reasons, including side effects and cost, tamoxifen may be a better choice for some women.

When you're deciding on a treatment plan, keep two things in mind:

  • Every cancer responds differently to treatment. What works for someone else may not work for you and what works for you may not work for someone else.
  • Your treatment plan isn't written in stone. You can switch medicines if another treatment has greater benefits and fewer side effects.

If you're being treated for hormone-receptor-positive breast cancer, talk to your doctor about the risks and benefits of tamoxifen and aromatase inhibitors. If you're currently taking tamoxifen, ask whether switching to an aromatase inhibitor makes sense for you. Together, you and your doctor can decide on a treatment plan that's the best one for you.

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