To determine if the Clarity 3DUS (3D Ultrasound) system can be used by multiple individuals to acquire 3DUS images and produce consistent TB (Tumor Bed) shifts for breast IGRT (Image-Guided Radiation Therapy).

TB Contouring The CT1, CT2, and US1 tumor beds will be manually contoured on the RM workstation by one of the study radiation therapists. Contouring will be performed using previously defined contouring guidelines. Available automatic contouring tools, including the 'interpolate' contouring function, may be utilized and contouring times will be recorded.

IGRT Assessment The patient will be set up on the breast board in treatment position. The 'ultra path' component of the Clarity system, which displays the path of the previous US acquired at CT simulation, will be used to guide the acquisition of US2. Although performed in the simulator room, the Clarity system will be used in 'treatment mode' thus simulating Clarity as it is used for IGRT. The study RT will first acquire the US2 scan. All steps following the acquisition of US2 will be performed off-line on the RM workstation without the need for further simulator or patient time.

Inclusion Criteria:

• Subjects will be 27 consenting women undergoing CT simulation for adjuvant breast radiotherapy at VIC ≤16 weeks after BCS.
• Women must have primary tumors <3cm, pathologically N0 disease, and negative surgical margins.
• Seroma clarity score <2

Exclusion Criteria:

• Patients will be excluded if they have had mastectomy or receive adjuvant chemotherapy that results in the CT simulation session occurring >16 weeks after the BCS date.