(HealthDay News) – For women with estrogen receptor (ER) positive or ER untested invasive breast cancer, continuing tamoxifen for 10 years is associated with significant reductions in recurrence and breast cancer mortality, according to a study presented at the annual meeting of the American Society of Clinical Oncology, held from May 31–June 4 in Chicago.

Richard G. Gray, from the University of Oxford in the United Kingdom, and colleagues examined the potential benefit of extending tamoxifen treatment to 10 years. A total of 6,953 women with ER positive or ER untested invasive breast cancer from 176 UK centers were randomized to stop tamoxifen after five years or continue for an additional five years. Compliance, recurrence, mortality, and hospital admissions were recorded at annual follow-ups.

The researchers observed a significant reduction in breast cancer recurrence with allocation to continue tamoxifen, which was time dependent (rate ratio, 0.99 during years five to six; 0.84 during years seven to nine; and 0.75 later). Breast cancer mortality was significantly reduced with longer treatment (P=0.05; rate ratio, 0.77 after year nine), and overall mortality was also reduced (P=0.1; rate ratio, 0.86 after year nine), but there was little effect on non-breast-cancer mortality. There was a significantly increased risk of endometrial cancers with longer tamoxifen treatment (rate ratio, 2.2).

“Until now, though, there have been doubts whether continuing tamoxifen beyond five years is worthwhile,” Gray said in a statement. “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.”

More Information