This study's primary aim is to determine rates of recurrence with the innovative approach of considering tumor biology to select patients who may avoid radiation, with restriction of eligibility to women aged 50-69 with hormone-sensitive, Her2-negative tumors with Oncotype-DX RS ≤ 18 who plan to receive endocrine therapy. In this way, this study seeks to collect prospective data supporting the idea that this is a population at low risk of LRR in whom omission of adjuvant radiation is reasonable.