A question of duration: do patients with early-stage breast cancer need more than five years of adjuvant endocrine therapy?

Women with hormone receptor-positive breast cancer continue to be at risk for recurrence and mortality for many years after diagnosis. Previous clinical trials established 5 years of endocrine therapy as a standard of care for both premenopausal and postmenopausal women, resulting in long-lasting benefit over shorter durations of treatment. Until recently, trials testing durations of tamoxifen longer than 5 years have not shown additional benefit, but the ATLAS (Adjuvant Tamoxifen, Longer Against Shorter) trial, reported in 2007, showed a small but significant reduction in risk of recurrence with 10 compared with 5 years of tamoxifen therapy. Aromatase inhibitors (AIs) improve relapse-free survival (RFS) in postmenopausal women when they are used sequentially with, or replace, tamoxifen for a total of 5 years of therapy. Extension of endocrine therapy to 10 years in the National Cancer Institute of Canada Clinical Trials Group MA.17 trial demonstrated that 5 years of letrozole therapy following 5 years of tamoxifen therapy results in an improvement in RFS, but not overall survival, in postmenopausal women. Trials testing durations of AI therapy for longer than 5 years are ongoing. Selection of candidates for extended endocrine therapy should balance recurrence risk, toxicity of treatment, and comorbidities that might impact life expectancy and risk of side effects.