— Objective: Lumpectomy, breast conserving tumor excision, is the standard surgical treatment in early stage breast cancer. A common problem with lumpectomy is that the tumor may not be completely excised, and additional surgery becomes necessary. We investigated if a surgical navigation system using intraoperative ultrasound improves the outcomes of lumpectomy, and if such a system can be implemented in the clinical environment. Methods: Position sensors were applied on the tumor localization needle, the ultrasound probe, and the cautery, and three-dimensional navigation views were generated using real time tracking information. The system was tested against standard wire-localization procedures on phantom breast models by eight surgical residents. Clinical safety and feasibility was tested in six palpable tumor patients undergoing lumpectomy by two experienced surgical oncologists. Results: Navigation resulted in significantly less tissue excised compared to control procedures (10....