How do you explain the benefit of endocrine therapy after bilateral mastectomies in early stage breast cancer for a postmenopausal woman?

First of all, all breast cancer is considered to have seeded systemic mets. Also, early is a concept mainly for surgery and radiation while it is important to remember that single cell that became malignant took 7-10 years to reach 1 cm stage if ER positive, and it is to be presumed that some cells have escaped local domain and are at risk of systemic relapse. Hence, endocrine therapy is essential even after bilateral mastectomies to increase chance for cure.