Demographic data of the study cohort.

Background
Numerous clinical trials have compared the efficacy of endocrine therapy in post-menopausal breast cancer patients. This study aims to explore whether there is a difference in recurrence rates between this population using tamoxifen and aromatase inhibitors (AIs) by analyzing real-world data.
Methods
This retrospective cohort study utilized the National Health Insurance (NHI) claims data and the Taiwan Cancer Registry (TCR). We identified 6,050 patients aged over 55 diagnosed with ER-positive, HER2-negative early breast cancer between 2012 and 2016 (4,451 on AIs alone and 1,599 on tamoxifen alone). Recurrence in both groups was assessed until the end of 2020. Hazards were measured based on age of diagnosis, cancer stage, adjuvant chemotherapy, radiation therapy, type of endocrine therapy used, and adherence. Recurrence‑free survival between the AIs and tamoxifen groups was evaluated using the Kaplan-Meier model.
Results
The average age was 65.1 years, with a median follow-up time of 5.7 years and a median duration of endocrine therapy of 4.5 years. The recurrence rate was 2.2%. Using tamoxifen as endocrine therapy reduces the risk of recurrence compared to AIs (adjusted HR: 0.32, p < 0.0001). There was no statistical difference between the two drugs in stage 1 breast cancer. However, in stage 2, the risk of breast cancer recurrence decreased to 0.15 times with the use of tamoxifen compared to AIs (p = 0.0002). Stage 2 cancer, histological grade 3, and non-adherence increased recurrence risk in post-menopausal breast cancer patients.
Conclusion
Based on real-world data analysis, in ER-positive, HER2-negative post-menopausal women with early breast cancer in Taiwan, the use of tamoxifen compared to AIs is associated with a lower risk of recurrence. Improved adherence to medication can break the cycle of recurrence and improve health outcomes.