Extended adjuvant endocrine therapy in hormone-receptor positive breast cancer.

A high ongoing recurrence rate in patients with endocrine responsive breast cancer provides the rationale for offering endocrine treatment for more than five years. The MA.17 study, comparing the aromatase inhibitor (AI) letrozole for five years after an initial five years of tamoxifen to no further treatment, provided the proof-of-principle for extended endocrine treatment. These results have meanwhile been confirmed by several other studies and an EBCTCG meta-analysis. More recently, data from the ATLAS trial, comparing 10 to five years of tamoxifen, have been published, similarly showing a benefit for longer endocrine treatment with tamoxifen. In postmenopausal women -including those who had been premenopausal at initial diagnosis - a cross-trial comparison of ATLAS and the AI studies indicates superiority of switching to letrozole versus ongoing tamoxifen, similar to superiority of the AIs over tamoxifen in the metastatic and early breast cancer settings.