August 2013 Vol 6, No 6 Special Issue - Breast Cancer
Wayne Kuznar

Chicago, IL—Updated data confirm that 10 years of adjuvant tamoxifen is superior to 5 years in reducing the rates of late recurrence and death in women with estrogen receptor (ER)-­positive breast cancer, reported Richard G. Gray, MA, MSc, Professor of Medical Statistics, University of Oxford, United Kingdom.

Previously, 5 years of treatment with tamoxifen had been shown to reduce breast cancer mortality by approximately 33% over 15 years. At ASCO 2013, Mr Gray presented data from a study known as Adjuvant Tamoxifen Treatment—To Offer More? (aTTom), which demonstrated an additional 25% reduction in cancer recurrence and death rates from year 10 onward in women treated with tamoxifen for 10 years compared with for 5 years. These results complement and confirm the results from the recently released and published study known as ATLAS (Adjuvant Tamoxifen: Longer Against Shorter; Davies C, et al. Lancet. 2013;381:805-816).

In the aTTom study, 6953 women in the United Kingdom who had been taking tamoxifen for 5 years were randomized to continuing treatment for an additional 5 years or to stopping treatment. Approximately 75% of the women assigned to continue tamoxifen had done so for 5 extra years.
Treatment with tamoxifen for 10 years compared with 5 years reduced cancer recurrence by 15% (P = .003). The rate of deaths from breast cancer was reduced by 12% with 10 years of treatment with tamoxifen relative to 5 years (P = .06).

“Treatment allocation had little effect on either recurrence rates or death rates from 5 to 9 years after diagnosis, but the benefit of longer treatment became evident in the second decade after diagnosis,” said Mr Gray. The relative reduction in the risk of death with 10 years of tamoxifen therapy increased to 21% in years 10 to 14 after diagnosis, and to 25% by 15 years of follow-up and later.

No significant differences were seen in death without recurrence or all-cause mortality rates between the 2 groups, although a significant improvement (P = .016) in overall survival emerged from 10 years onward of tamoxifen therapy.

Extending the use of tamoxifen approximately doubled the risk of developing endometrial cancer (from 1.3% to 2.9%) and of death from endometrial cancer (from 0.6% to 1.1%), but the net clinical benefit favored 10 years of treatment, he said.

Compared with no tamoxifen treatment, 10 years of tamoxifen therapy reduces the rate of breast cancer death by approximately 33% in the first 10 years after diagnosis and by 50% subsequently, said Mr Gray. The benefit of tamoxifen as shown in the aTTom study may be even greater than what was reported, because 60% of enrolled patients had an unknown ER status. An estimated 15% of patients likely had ER-negative disease and did not benefit from treatment with tamoxifen.

These findings complement and confirm the results from the ATLAS study. The combined data from the aTTom and ATLAS studies, which included data from >17,000 patients, show a significant 15% reduction in breast cancer mortality overall (P = .001) and an additional 25% reduction in breast cancer mortality 10 years and beyond after 10 years compared with 5 years of treatment with tamoxifen (P = .004).

Together, the results of the aTTom and ATLAS trials constitute “proof beyond reasonable doubt” that continuing tamoxifen beyond 5 years reduces the risk of late breast cancer recurrence and reduces breast cancer mortality, said Mr Gray.

Late relapses remain a concern in patients with ER-positive breast cancer, even with the introduction of effective early therapies, said Ann H. Partridge, MD, MPH, Medical Oncologist, Dana-Farber Cancer Institute, Boston, and extended treatment with tamoxifen appears to reduce the risk of late relapses. In the aTTom and the ATLAS studies, extending tamoxifen primarily reduced the risk of breast cancer recurrences starting after year 7.

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