The purpose of this study is to evaluate an investigational way to provide radiation therapy to treat early-stage breast cancer at the time of surgery.

This is a prospective single-arm phase II study designed to study the efficacy of high dose rate (HDR) brachytherapy with CT-imaging, single-fraction intraoperative radiation therapy (IORT). The study will accrue patients with early-stage breast cancer who will be treated with breast-conserving surgery (BCS). The main clinical eligibility criterion is based on suitability for accelerated partial breast irradiation. Patients may be eligible prior to receiving BCS or within 30 days of receiving BCS. Eligible patients that will undergo BCS will receive surgery according to the standard clinical practice in the Brachytherapy Suite in the Emily Couric Clinical Cancer Center. Patients that have already undergone BCS must meet all other eligibility criteria and are able to be treated with IORT within 30 days of their breast surgery. Eligible subjects will undergo a lumpectomy or re-excision lumpectomy according the standard clinical practice. Immediately following lumpectomy, the breast HDR brachytherapy applicator will be placed and CT imaging will be performed for treatment planning. After HDR brachytherapy is delivered, the applicator will be removed, and the breast surgeon will complete the skin closure. Although the ultimate goal of the study is to determine 5-year ipsilateral breast tumor recurrence (IBTR), all efforts will be made to follow the patients until death.

Inclusion Criteria:

1. Patient has elected breast conserving surgery or whole breast irradiation treatment for early-stage breast cancer.
2. Tumor size must be less than or equal to 3 cm.
3. Patient is 45 years of age or older.

Exclusion Criteria:

1. Male patients.
2. Pregnant patients.
3. Breast cancer that involves the skin or chest wall.
4. History of ipsilateral breast cancer.
5. Multicentric breast cancer in the ipsilateral breast.
6. Known BRCA gene mutation.
7. Patient with nodal disease.
8. Patient is undergoing initial medical treatment (hormone or chemotherapy) to reduce tumor size.
9. Patient with breast implants (does not include patients having implants AFTER intra-operative radiotherapy).