While some types of cancer can be considered to be cured after five years without a relapse, this is not the case for breast cancer. Breast cancer has a long natural history and can recur many years after initial diagnosis and treatment. Women with aggressive disease such as triple negative (ER-/PR-/HER2-), HER2 positive (HER2+) or inflammatory breast cancer (IBC) are most likely to relapse within the first two to four years after diagnosis. On the other hand, the majority of later recurrences are hormone receptor positive and HER2 negative (ER+/PR+/HER2-), a subtype generally considered to be far less aggressive.

The average risk of recurrence between years five to 12 after initial breast cancer surgery is approximately 4% per year, according to one study. Other studies have found that the rate of recurrence for early-stage breast cancer is 1% to 2% overall after the first 10 years. While the risk of recurrence is lower at first for ER+ compared to ER- disease, ER+ tumor status is less favorable after approximately 7.7 years, according to another study. Although the rate of recurrence declines over time, breast cancer has been known to recur decades after the original treatment.

The few studies that have examined the topic have not identified prognostic factors consistently associated with very late recurrence. For example, high proliferation (as measured by Ki-67) predicts early but not late risk of recurrence. Positive hormone receptor status appears to be the only attribute consistently associated with such recurrence. Now a new study has reported that treatment with tamoxifen for 10 years (rather than five) helps prevent late recurrence in hormone receptor positive breast cancer patients.

Latest research reports that 10-year tamoxifen treatment has survival benefit

The study referenced at the beginning of this news article was designed to investigate the effects of continuing tamoxifen for 10 years instead of ending it after five years as is customary. Five years of tamoxifen treatment has been shown to significantly lower breast cancer mortality among women with early-stage ER+ breast cancer during the first 15 years after diagnosis. The study included women enrolled in the worldwide Adjuvant Tamoxifen: Longer Against Shorter (ATLAS) trial. Study participants entered the trial between 1996 and 2005. The women completed five years of tamoxifen treatment, then were randomly assigned either to continue tamoxifen to the 10-year mark or not.

The current report describes outcomes to date among 6,846 women with ER+ breast cancer and some other women with ER- or undetermined ER status. Study participants continue to be followed. Women with ER+ disease who were allocated to continue tamoxifen were found to have reduced risk of recurrence (18.0% recurrence rate) compared to those who stopped at five years (20.8%). In addition, women who continued on tamoxifen experienced a lower breast cancer-specific death rate (9.7% compared to 11.6%) and reduced death rate from any cause (18.6% compared to 21.1%).

The positive effects of continuing tamoxifen rather than stopping at five years were found to be particularly strong after year 10. In fact, the additional treatment was found to have had little effect on recurrence during the period five to nine years after diagnosis. However, during the second decade after diagnosis, study participants who continued tamoxifen treatment were found to have a 25% lower recurrence rate and a 29% lower breast cancer-specific death rate compared to those who stopped after five years.

Although continuing tamoxifen increased the risk of endometrial cancer (and other side effects), the authors judged this risk acceptable given the curability of early-stage endometrial cancer compared to breast cancer recurrence. The cumulative risk of endometrial cancer during years five to 14 was 3.1% (mortality 0.4%) for women allocated to continue tamoxifen compared to 1.6% (mortality 0.2%) for those who stopped after five years. In premenopausal women, for whom tamoxifen is often the endocrine treatment of choice, there was no apparent increase in endometrial cancer. The authors conclude that the study results, taken together with results from previous trials of five 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.

Please see our articles on the impact of endocrine therapy on breast cancer prognosis and what to eat during tamoxifen treatment for more information.