A trial looking at 5 years or 10 years of tamoxifen for women with early breast cancer (aTTom)

The team found that taking tamoxifen for 10 years reduces the risk of breast cancer coming back more than taking it for 5 years.

Trial design
This trial was for women who’d had surgery for early stage breast cancer. They were already taking tamoxifen to help stop their cancer coming back.

There were 2 treatment groups. One group took tamoxifen for 5 years. The other took tamoxifen for 10 years.

Results
A total of 6,953 women joined this trial. They were put into 1 of 2 treatment groups at random:

• 3,485 had tamoxifen for 5 years
• 3,468 had tamoxifen for 10 years

Diagram 1

How well treatment worked
In 2013, the research team presented results about how well 10 years of tamoxifen works.

They looked at how many women’s breast cancer had come back. They found it was lower in the group who took tamoxifen for 10 years:

• 672 women (19%) who took tamoxifen for 5 years
• 580 women (17%) who took tamoxifen for 10 years

Bar Chart 1

They also looked at the number of women who had died. This includes women who had died because of their breast cancer, and those who died of other causes. 

They found this was also lower in the group who took tamoxifen for 10 years:

• 910 women (26%) who took tamoxifen for 5 years
• 849 women (24%) who took tamoxifen for 10 years

Bar Chart 2

Womb cancer
Taking tamoxifen can increase the risk of developing womb (endometrial) cancer. The number of women who developed womb cancer was low, but higher in those who took tamoxifen for longer:

• 45 women (1.3%) who took tamoxifen for 5 years
• 102 women (2.9%) who took tamoxifen for 10 years

They also looked at the number of women who died from womb cancer. It was:

• 20 women (0.6%) who took tamoxifen for 5 years
• 37 women (1.1%) who took tamoxifen for 10 years

Who is likely to benefit from more tamoxifen
In 2019 and 2022, the trial team published results about who is most likely to benefit from taking tamoxifen for longer. This part of the trial is called Trans-aTTom.

They used something called the Breast Cancer Index (BCI). This looks at 11 genes known to affect breast cancer risk. It gives a score based on how many of these genes have changes (mutations).

The team looked at the genes of 789 women who had breast cancer that had spread to their lymph nodes . They used samples of cancer tissue that had been taken during surgery.

They found that people with a higher BCI score were more likely to benefit from 10 years of tamoxifen. 

aTTom-Extended
The research team wanted to find out more about the effects of taking tamoxifen for longer. They aim to do this with a part of the trial called aTTom-Extended. 

They have collected cell (tissue) samples from people who took part. These have now been stored for future ethically approved research. The team will also use medical records to find out how people are doing, up until 2027.

If you took part in the aTTom trial but do not want the researchers to continue to collect data about you or use your stored tissue samples for further research, you can contact them to let them know. You can email the trial team on: 
aTTom-extended@trials.bham.ac.uk 

We hope to add results from the longer term follow-up to this page once they are available.

Conclusion
The trial team concluded that taking tamoxifen for 10 years reduces the risk of breast cancer coming back more than taking it for 5 years. They suggest that this benefit outweighs the risk of developing womb cancer.

They also concluded that the Breast Cancer Index score could be useful to help predict who is most likely to benefit from taking tamoxifen for longer.

More detailed information
There is more information about this research in the references below. 

Please note, the articles we link to below are not in plain English. They have been written for healthcare professionals and researchers.

aTTom: Long-term effects of continuing adjuvant tamoxifen to 10 years versus stopping at 5 years in 6,953 women with early breast cancer
Richard G Gray and others
Journal of Clinical Oncology, 2013. Volume 31, issue 18 supplement.

Breast Cancer Index and prediction of benefit from extended endocrine therapy in breast cancer patients treated in the Adjuvant Tamoxifen—To Offer More? (aTTom) trial
J M S Bartlett and others
Ann Oncol, 2019. Volume 30, issue 11, pages 1776 to 1783.

Correlative studies of the Breast Cancer Index (HOXB13/IL17BR) and ER, PR, AR, AR/ER ratio and Ki67 for prediction of extended endocrine therapy benefit: a Trans-aTTom study
Dennis C Sgroi and others
Breast Cancer Research, 2022. Volume 24, issue 90.

Where this information comes from    
We have based this summary on the information in the articles above. These have been reviewed by independent specialists (peer reviewed ) and published in medical journals. We have not analysed the data ourselves. As far as we are aware, the links we list above are active and the articles are free and available to view.