(ChemotherapyAdvisor) – Continuing adjuvant tamoxifen to 10 years—rather than stopping at the usual 5 years—further reduces the likelihood of recurrence and death from estrogen-receptor (ER)–positive breast cancer, particularly after year 10, results of Adjuvant Tamoxifen: Longer Against Shorter (ATLAS) trial concluded in The Lancet.

Published to coincide with a presentation at the 2012 CTRC-AACR San Antonio Breast Cancer Symposium, “these results, taken together with results from previous trials of 5 years of tamoxifen treatment versus none, suggest that 10 years of tamoxifen treatment can approximately halve breast cancer mortality during the second decade after diagnosis,” said Christina Davies, BMBCh MSc, of the Clinical Trial Service Unit and Epidemiological Studies Unit (CTSU) at the University of Oxford, UK.

Between 1996 and 2005, the ATLAS (Adjuvant Tamoxifen: Longer Against Shorter) trial randomly assigned 12,894 women with early breast cancer who had completed 5 years of treatment with tamoxifen to continue the agent to 10 years or stop at 5 years.

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Among 6,846 women with ER-positive disease, 617 of 3,428 women who continued to receive tamoxifen had a reduced risk of breast cancer recurrences, 711 of 3,418 in the control arm (P=0.002); reduced breast cancer mortality, 331 deaths vs 397 deaths (P=0.01), and reduced overall mortality, 639 deaths vs 722 deaths (P=0.01), respectively, were also noted.

“The reductions in adverse breast cancer outcomes appeared to be less extreme before than after year 10,” Dr. Davies reported. The recurrence rate ratio (RR) was 0.90 (95% CI 0.79–1.02] during years 5-9 and 0.75 (95% CI 0.62–0.90) in later years; for breast cancer mortality, the RR was 0.97 (95% CI 0.79–1.18) during years 5-9 and 0.71 (95% CI 0.58–0.88) in later years.

During years 5-14, cumulative risk of recurrence was 21.4% for women allocated to continue tamoxifen vs 25.1% for controls; for breast cancer mortality, it was 12.2% vs 15.0%, respectively, for an absolute mortality reduction of 2.8%.

No effect on breast cancer outcome was observed among the 1,248 women with ER-negative disease; an intermediate effect was seen among 4,800 women with unknown ER status.

“Among all 12,894 women, mortality without recurrence from causes other than breast cancer was little affected,” she noted. An increase was seen in endometrial cancer; excess risk of dying by year 15 was 0.4% in the tamoxifen arm vs 0.2% in the control arm.

Long-term follow-up continues.