Our proposal aims to improve and individualize the treatment of early-stage breast cancer in women aged 65 years and older with a novel use of a three-month course of pre-operative endocrine therapy (pre-ET). Early- stage breast cancer is traditionally treated with breast conserving surgery (BCS) followed by adjuvant radiation therapy (RT) and a 5- to10-year course of adjuvant endocrine therapy (AET). Long-term data from two randomized controlled trials demonstrate that omission of RT in older women with estrogen receptor positive, early-stage breast cancer who are treated with AET does not result in a decreased survival. However, over 70% of women still receive RT, due in part to physician and patient reluctance to omit a treatment in light of concerns about AET toxicity and adherence. Optimal choices for older breast cancer patients require considering data-based recommendations, life expectancy, co-morbidities and patient preferences. We will evaluate the use of pre-ET as a novel experiential tool to inform adjuvant therapy decisions, predict long-term AET adherence and to reduce decisional conflict and regret, addressing the following specific aims: Aim 1: To assess whether pre-ET changes patients’ and treating physicians’ preference for adjuvant RT. This test exposure will provide patients with the information they need to choose among the treatment options after BCS. We hypothesize that a 3-month course of pre-ET will reduce patients’ and physicians’ preferences for adjuvant RT by 10% from baseline preferences. We will also assess whether patient-reported outcomes during pre-ET predict AET adherence at 2 years. We will obtain exploratory data on tumor response to pre-ET by evaluating tumor proliferation indices before and after pre-ET. Aim 2: To measure the effect of pre-ET on decisional outcomes for adjuvant therapy after BCS. We will apply the Ottawa Decision Support Framework to evaluate the effects of a trial of endocrine therapy on decisional conflict and decisional regret, framing pre-ET as a decision support intervention. We will measure decisional regret at 1 and 2 years after BCS. We hypothesize that pre-ET will reduce decisional conflict and regret, and that decisional conflict and regret scores will be associated with 1- and 2-year adherence to AET. The use of pre-ET in the proposal represents a patient-centered treatment approach and may ultimately lead to a paradigm shift in the way that early-stage breast cancer is treated in older women, allowing for optimization assignment of adjuvant therapy, increased adherence to planned treatment regimens and decreased decisional regret.