After completion of preoperative neoadjuvant systemic chemotherapy and on the day of definitive breast cancer surgery, patients will be injected intradermally with approximately 400 microcuries of Tc-99m sulfur colloid. Patients will undergo preoperative lymphoscintigraphy in the nuclear medicine department to access axillary and extra-axillary sites of localization. Intraoperatively, patients will also be injected with approximately 4 to 5 mL of 1% isosulfan blue dye (by intradermal, intraparenchymal, or subareolar route). Patients will undergo intraoperative identification and biopsy of all SLN candidates. A confirmatory axillary lymph node dissection will then be performed on all patients.