All patients will undergo a partial mastectomy with sentinel lymph node biopsy with the goal of achieving a margin-negative resection while maintaining good cosmetic outcome. Immediately following partial mastectomy and frozen section evaluation of the sentinel lymph nodes, IORT is to be delivered. Intraoperative radiation therapy will involve 50 kV xrays to a dose of 20 Gy during breast conserving surgery. After surgery, patients are followed based on the standard schedule determined by their surgeon for 5 years.

Intraoperative radiation therapy with INTRABEAM system to a dose of 20 Gy

Ipsilateral In-Breast Tumor Recurrence (IBTR): defined as biopsy-proved invasive or in situ breast cancer (except LCIS) in the ipsilateral breast.

Degree of late radiation toxicity will be scored based upon RTOG/EORTC (Radiation Therapy Oncology Group/Eastern Oncology Radiation Therapy Consortium) late radiation morbidity scoring schema for subcutaneous tissue as well as cosmetic result.

The RTOG/EORTC late radiation morbidity scoring schema on subcutaneous tissue is as follows:

• GRADE 1: Slight induration (fibrosis) and loss of subcutaneous fat
• GRADE 2: Moderate fibrosis but asymptomatic; slight field contracture; <10% linear reduction
• GRADE 3: Severe induration and loss of subcutaneous tissue; field contracture > 10% linear measurement
• GRADE 4: Necrosis