Estrogen therapy after breast cancer diagnosis and breast cancer mortality risk

Purpose The safety of local estrogen therapy in patients on adjuvant endocrine treatment is questioned, but evidence on the issue is scarce. This nested case–control registry-based study aimed to investigate whether estrogen therapy affects breast cancer mortality risk in women on adjuvant endocrine treatment. Methods In a cohort of 15,198 women diagnosed with early hormone receptor (HR)-positive breast cancer and adjuvant endocrine treatment, 1262 women died due to breast cancer and were identified as cases. Each case was matched with 10 controls. Exposure to estrogen therapy with concurrent use of aromatase inhibitors (AIs), tamoxifen, or both sequentially, was compared between cases and controls. Results No statistically significant difference in breast cancer mortality risk was seen in patients with exposure to estrogen therapy concurrent to endocrine treatment, neither in short-term or in long-term estrogen therapy use. Conclusions The study strengthens current evidence on local estrogen therapy use in breast cancer survivors, showing no increased risk for breast cancer mortality in patients on adjuvant AIs or tamoxifen.