Maria Sund, et al (Breast Cancer Research and Treatment volume 198, pages361–368 (2023) ) investigated whether the safety of local estrogen therapy in patients on adjuvant endocrine treatment using a nested case–control registry-based study 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.
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.