HealthDay News — For women with estrogen receptor-positive breast cancer, continuing tamoxifen for 10 years instead of five was associated with significant reductions in breast cancer recurrence and mortality, study results indicate.

Richard Gray, MSc, of the University of Oxford in the United Kingdom, presented results from the aTTom trial, a phase III randomized study, at the American Society of Clinical Oncology (ASCO) 2013 meeting in Chicago. The findings are similar to results from another study, the ATLAS trial, that Gray presented earlier this year at the San Antonio Breast Cancer Symposium.

“Until now, there have been doubts whether continuing tamoxifen beyond five years is worthwhile,” he said. “This study, and its international counterpart ATLAS, confirm that there is definitely a survival benefit from longer tamoxifen treatment and many doctors will likely recommend continuing tamoxifen for an extra five years.”

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The extension also increased side effects, specifically an increased risk for endometrial cancer, but overall the benefits outweighed the risks, according to Gray.

From 1991-2005, the researchers randomly assigned 6,953 women with ER-positive or ER-untested invasive breast cancer from 176 U.K. centers to either stop tamoxifen after five years or continue for an additional five years to assess the potential benefit of a 10 year regimen. Compliance, recurrence, mortality, and hospital admissions were recorded at annual follow-ups.

They found that among the 3,468 women who continued therapy, there were 580 cancer recurrences, compared with 672 among the 3,485 women who stopped, a difference that was significant (P=0.003). The risk reduction was greater the longer the duration of tamixofen, with a rate ratio of 0.99 during years five to six, 0.84 during years seven to nine, and 0.75 for 10 years.

Longer treatment also reduced breast cancer mortality, with 392 deaths versus 443 deaths after recurrence (P=0.05; RR, 0.77 after year 9).

There were more endometrial cancers among patients assigned to the extended regimen (102 vs. 45, P<0.001) as well as more deaths from endometrial cancer (37 vs. 20, P=0.02). However, it is estimated that long-term tamoxifen would prevent 30 breast cancer deaths for every endometrial cancer death that occurs as a side effect of the extra use of the drug, according to the researchers.

They observed no effect on overall mortality or nonbreast cancer mortality with extended tamoxifen.


  1. Gray RG et al. Abstract #5. “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: ASCO 2013. May 31-June 4, 2013. Chicago.