The IDEA Study (Individualized Decisions for Endocrine Therapy Alone)

Inclusion Criteria: * Postmenopausal status, as defined by (a) patients age 60 or greater, or, (b) patients age 50-69 with either (a) s/p bilateral oophorectomy, or, (b) intact uterus without menses in the past 12 months, or, (c) biochemical confirmation of postmenopausal status. * Histopathological confirmation of Stage 1 (pT1N0M0) invasive breast cancer status post breast conserving surgery * Negative axillary nodes (isolated tumor cells with no cluster measuring \>0.2mm allowed) * Allowable options for axillary staging include: * Sentinel node biopsy only * Sentinel node biopsy followed by axillary dissection * Axillary dissection only * Margins of excision ≥2mm * ER+, PR+, Her2 - using the current College of American Pathologists guidelines * Oncotype-DX RS ≤ 18 * Disease must be unifocal on clinical, radiologic, and pathologic examination * Registration within 90 days of last surgical procedure for breast cancer treatment * Patient must willingly sign study specific informed consent prior to study entry * Patient must be a candidate for and willing to take endocrine therapy for minimum of 5 years (aromatase inhibitor or tamoxifen). Patients who have already begun endocrine therapy after lumpectomy are eligible. * Patient must have had breast imaging (mammogram or MRI) of the ipsilateral breast within 6 months and contralateral breast within 1 year of study entry. * Patient must have Zubrod performance status 0-2 Exclusion Criteria: * Evidence of multifocal or…