• The patient must sign consent for study participation.

• The patient must be female and have a diagnosis of an invasive or non-invasive breast cancer that was treated surgically by a partial mastectomy.

• The patient must be deemed an appropriate candidate for breast conserving therapy (i.e. not pregnant, never had radiation to the treated breast, breast size would allow adequate cosmesis after volume loss from partial mastectomy).

• Patients with involved lymph nodes are candidates for the study.

• Surgical margins are negative for invasive (no tumor on ink) or non-invasive breast cancer (2 mm negative margin).

• The greatest dimension of the tumor is less than 4cm before surgery.

• Multifocal disease is allowed if it was removed by a single lumpectomy resection and the patient remained a candidate for breast conservation.

• Age 18 years and older.

• Women of childbearing potential (pre-menopausal defined as having a menstrual period within the past 1 year) must have a negative serum pregnancy test or complete a pregnancy waiver form per institutional policy.

• The surgical cavity is clearly visible on CT images. Of note, clips are not required but recommended.

• The patient must weigh less than 150Kg (330lb), which is the limit of the imaging couch.

• The patient must be less than 6'6 in height.

• The patient must feel comfortable in the prone position.

• Diagnosis of prior contralateral breast cancer is allowed.

• Diagnosis of synchronous bilateral cancers is allowed. In this case if bilateral boosts are required, a patient would not have both treatments on the same day.

• Oncoplastic reduction surgery is allowed if the lumpectomy cavity can be clearly visualized.