Treatment adoption and relative effectiveness of aromatase inhibitors compared to tamoxifen in early breast cancer: A multi-institutional observational study

Background Since 2005, aromatase inhibitors (AIs) have been the adjuvant treatment of choice for postmenopausal women with early breast cancer (BC). In this study we characterize the adoption of AIs in Portugal, variables associated with treatment administration, and compare its effectiveness (either in monotherapy or sequential therapy) to tamoxifen monotherapy (TAM). Patients and methods This was a retrospective cohort study that included postmenopausal women with stage I-III hormone receptor (HR) positive BC diagnosed from 2006 to 2008 and treated with adjuvant endocrine therapy in four participating institutions. Results Of the 1283 eligible patients, 527 (41%) received an AI (16% as monotherapy, 25% as sequential therapy) and 756 (59%) TAM. Patients treated with AI had less differentiated tumors, with higher TNM stage, and were more frequently HER2-positive. Use of AI also differed by center (use range from 33% to 75%, p < 0.001). With a median follow-up of 6.3 years and controlling for clinicopathological and treatment characteristics, treatment with AI had a better overall survival (OS) when compared with TAM (adjusted-HR 0.55, 95% CI 0.37–0.81). Conclusion AIs were successfully introduced as adjuvant treatment for HR-positive BC in Portuguese hospitals. Its use was influenced by tumor and patient characteristics, but also center of care. In this large cohort, AI use was associated with an OS benefit.