479 © Springer Nature Switzerland AG 2019 G. Novita et al. (eds.), Breast Diseases, https://doi.org/10.1007/978-3-030-13636-9_58 Adjuvant Hormone Therapy for Breast Cancer Andrea K. Shimada, João Victor Machado Alessi, Bruna Zucchetti, and Artur Katz Introduction Estrogen plays a key role in the development of breast cancer, and its blockage is the basis of hormone therapy. Hormone treatment is the cornerstone of adjuvant systemic treatment based on its benefts and the number of patients who are candidates for it. A Few Background Facts More than a century ago, Beatson frst showed the dependence of estrogen on the genesis of breast cancer. Several studies and meta-analyses have demonstrated the overall survival beneft of endocrine therapy in the adjuvant setting in patients with hormone receptor posi- tive tumours (HR+) associated with a mild and manageable toxicity profle. In this context, the agents employed are selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AI). A. K. Shimada · J. V. M. Alessi · B. Zucchetti · A. Katz (*) Clinical Oncology, Sírio-Libanês Hospital, São Paulo, SP, Brazil