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For women with early-stage, estrogen receptor-positive (ER-positive) breast cancer, ten years of adjuvant tamoxifen appears to be more effective than the standard five years. These results were presented at the 2013 Annual Meeting of the American Society of Clinical Oncology (ASCO).

Tamoxifen is a hormonal therapy that interferes with the effects of estrogen. It is commonly used in the treatment of ER-positive breast cancer, and is also used to reduce the risk of breast cancer in women at high risk of the disease. For women with early-stage breast cancer, that standard duration of treatment with tamoxifen is five years.

To assess whether ten years of treatment with tamoxifen is more effective than five years of treatment, researchers in the UK conducted a Phase III clinical trial (the aTTom trial).[i] The study enrolled 6,953 women with ER-positive breast cancer. After five years of treatment with tamoxifen, women were randomly assigned to either stop taking tamoxifen or to continue it for another five years.

A majority of women have now been followed for more than 10 years, and some have been followed for up to 20 years.

  • 10 years of tamoxifen reduced the risk of breast cancer recurrence. A recurrence occurred in 16.7% of women treated with 10 years of tamoxifen and 19.3% of women treated with 5 years of tamoxifen. Longer-term treatment also reduced the risk of dying of breast cancer. These benefits became apparent during the second decade after breast cancer diagnosis.
  • Longer-term treatment with tamoxifen did increase the risk of endometrial cancer (cancer of the lining of the uterus). Endometrial cancer can often be detected early. Furthermore, the researchers estimated that for every endometrial cancer death that occurred with longer-term tamoxifen, 30 breast cancer deaths would be prevented. The researchers concluded that the benefits of longer-term tamoxifen greatly outweigh the risks.
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These results are consistent with another large clinical trial, the ATLAS study, which also found a benefit of longer-term tamoxifen.[ii] Together, these studies indicate that 10 years of tamoxifen is more effective than 5 years of tamoxifen for women with early-stage, ER-positive breast cancer.


[i] Gray RG, Rea D, Handley K et al. aTTom: Long-term effects of continuing adjuvant tamoxifen to 10 years versus stopping at 5 in 6,953 women with early breast cancer. Presented at the 49th Annual Meeting of the American Society of Clinical Oncology. May 31-June 4, 2013; Chicago, IL. Abstract 5​

[ii] Davies C, Pan H, Godwin J et al. Long-term effects of continuing adjuvant tamoxifen to 10 years versus stopping at 5 years after diagnosis of oestrogen receptor-positive breast cancer: ATLAS, a randomized trial. Lancet. 2013;381:805-816.

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