British Study Confirms Benefit of Taking Tamoxifen for 10 Years

Breast Cancer News
August 13, 2013
Robin Warshaw, Contributing Writer
Reviewed By: 
Hope S. Rugo, MD

A study of women in the United Kingdom with estrogen receptor-positive (ER+), early-stage breast cancer showed that taking tamoxifen for 10 years reduced cancer return and deaths more than did stopping at the standard 5 years.

The findings, not yet published in a peer-reviewed journal, were presented at the American Society of Clinical Oncology (ASCO) 2013 annual meeting. Results showed benefits similar to those reported in a different, recent international study.


Tamoxifen is a medicine used to treat women with estrogen receptor-positive breast cancer. It blocks estrogen in some cells and reduces the risk of recurrence, or cancer returning in breasts and other organs.

Usually, tamoxifen is given to premenopausal women after surgery and chemotherapy (if necessary) as long-term adjuvant treatment. Sometimes, it is used as neoadjuvant therapy, before primary treatment.

Past studies first showed the benefit of taking tamoxifen for 1-, 2- and then 5 years. Until the findings of the international ATLAS study were published in 2012, showing decreases in breast cancer recurrence and death rates with 10 years compared with 5 years of use, there was not enough evidence to support the safety and benefit of giving women tamoxifen for longer than 5 years.

Reason for the Study

This study from Britain, known as aTTom (Adjuvant Tamoxifen – To Offer More?), was undertaken to find out if cancer recurrence and mortality could be decreased by extending the length of time women took tamoxifen from 5 to 10 years. Researchers also wanted to know if extending treatment would increase side effects.

Study Structure

From 1991–2005, nearly 7,000 U.K. women completing 5 years of tamoxifen treatment were randomly assigned to two groups. One group stopped taking tamoxifen after 5 years while the other continued the medicine for an additional 5 years (10 years total).

Most of the women had ER+ breast cancer, or were not tested but were estimated to be in that group (about 75 percent of breast cancers are ER+).

All of the women were followed annually to measure recurrences, serious side effects, hospital admissions and deaths. Data were compared between the two groups.


In contrast to the 3,485 women in the 5-year group, 3,468 women who took tamoxifen for 10 years had a

  • 25 percent lower rate of recurrence
    • 580 women in the 10-year group had a recurrence compared with 672 women in the 5-year group
  • 23 percent lower breast cancer mortality rate
    • Overall mortality of 849 women in 10-year group compared with 910 women in 5-year group

Both groups had similar rates of recurrence during years 5 through 9 after the start of tamoxifen.The benefits of extended treatment were seen in 10 years or more after starting tamoxifen. The majority of benefit in reduction of recurrence and death was seen after tamoxifen treatment had already been completed.

Overall, compared with taking no tamoxifen, 10 years of treatment reduced the number of deaths by about 1/3 during the first decade and by 1/2 after year 10.

More women (102, or 2.9 percent of 3,468 women) in the 10-year group developed endometrial cancer and 37 (1.1 percent) died, compared with women in the 5-year group (45, or 1.3 percent, diagnosed with endometrial cancer and 20, or .6 percent, died).

Those numbers are significant, yet endometrial cancer can be treated successfully if found early.

What This Means for You

If you are currently, or recently completed, taking tamoxifen for 5 years, you may be wondering if you should extend your treatment given the ATLAS and aTTom findings. Talk with your oncologist about what is best for you.

Whether you are given tamoxifen for longer than 5 years depends on several factors including your risk of recurrence, side effects you might experience, or plans to become pregnant. Keep in mind that tamoxifen has been proven to reduce the risk of recurrence and mortality from breast cancer, even when taken only for 5 years.

Your healthcare team can suggest ways to help you manage side effects such as hot flashes and vaginal dryness, should they arise during treatment. In rare cases, blood clots, stroke and endometrial cancer can occur.

You may want to read an LBBC Insight article on how other women considered their options for extending tamoxifen treatment. Remember, the treatment length that is right for another woman might not be right for you.

You can also talk with someone who understands by calling LBBC’s Breast Cancer Helpline, (888) 753-LBBC (5222).

Gray, RG, Rea, D, Handley, K, et al. aTTom: Long-term effects of continuing adjuvant tamoxifen to 10years versus stopping at 5 years in 6,953 women with early breast cancer. Journal of Clinical Oncology 31, 2013 (suppl; abstr 5).

This article was supported by Cooperative Agreement Number DP11-1111 from The Centers for Disease Control and Prevention. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the Centers for Disease Control and Prevention.

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