This study will collect rates of local/regional recurrence in select patients who do not receive radiation treatment after lumpectomy surgery. These women must be postmenopausal; have hormone receptor-positive, Her2-negative tumors; have Oncotype-DX RS less than or equal to 18; and plan to receive endocrine therapy. In this way, this study seeks to collect prospective data supporting the idea that this is a population at sufficiently low risk of local/regional recurrence that omission of adjuvant radiation might be a reasonable option.