Pilot study of the impact of letrozole vs. placebo on breast density in women completing 5 years of tamoxifen

Breast density, a strong risk factor for breast cancer, is reduced by the anti-estrogen, tamoxifen (TAM). We examined whether aromatase inhibitor (AI) therapy results in further reductions in breast density among women completing 5 years of TAM. Among a sample of women with early-onset breast cancer who were randomized to letrozole (LET)(n=56) or placebo (PLAC)(n=48) after 5 years of TAM, we examine the change in percent density at 9–15 months as well as a per-year change in PD by treatment group. There was no difference in the adjusted mean change (−1.0%, LET; −0.3%, PLAC (P=0.58)) or the percentage change (−2.7%, LET; −3.0%, PLAC (P=0.96)) in PD between treatment groups at 9–15 months. Results were similar for longitudinal change (−0.68% per year, LET; −0.12% per year, PLAC (P=0.23)). Breast density does not appear to be a clinically relevant biomarker in women who already have low PD following 5 years of TAM.