This is a multi-institutional, prospective, observational registry investigating the safety and efficacy of Accelerated Partial Breast Irradiation (APBI) using a Stereotactic Body Radiation Therapy (SBRT) delivery technique that incorporates real-time image guidance, noncoplanar fields, and respiratory tracking. The study will accrue 200 patients who will be treated to a dose of 30 Gy over the course of 5 fractions. These subjects will then have a follow up time of 5 years

RADIATION TREATMENT GUIDELINES and DOSIMETRY Fiducial Placement Prior to enrollment, patients will undergo breast conservation surgery preferably with fiducial markers implanted at the time of surgery or under ultrasonic guidance25

Patients treated with gantry-based SBRT preferably will have fiducials placed at the time of surgery.

For patients being treated with CyberKnife SBRT, four gold markers will be placed intraoperatively or ultrasonically to define the superior, inferior, medial, and lateral boundaries of the lumpectomy cavity in accordance with the manufacturer's (Accuray) documentation. At least three fiducials will be required for target tracking purposes to allow for rotational changes.

Simulation Linear Accelerator SBRT For treatments on a gantry-based linear accelerator, patients will be positioned supine with the arms elevated. CT scans will be obtained with 1.25 mm thickness and will be acquired with free breathing as well as additional scans as needed for motion control or gating. 4DCT, MRI and PET imaging may be used to assist in determining motion estimation.

CyberKnife SBRT For treatment on a robotic system, a contrast enhanced CT simulation is to be performed with 1 mm slices with the patient's arms at her sides in the supine position. The scan is to be performed with end-inspiratory breath hold from the thyroid to below the lungs. Breast immobilization devices including a vacuum mattress and a bra system will be allowed however bolus should not be used. Fiducial tracking (Synchrony) is to be used for motion tracking.

DOSIMETRY Target and Organs at Risk (OAR) contours

The following structures are to be contoured in all patients:

Tumor cavity (GTV) as defined on CT including all fiducials implanted on CT images Clinical Target Volume (CTV) defined as the tumor cavity expanded by a uniform 15 mm margin. The CTV will not extend beyond the skin, breast or chestwall Planning Tumor Volume (PTV) for SBRT will be defined as the CTV plus 3 - 5 mm as needed at the treating physician's discretion. PTV will be limited to GTV plus 20 mm margin Skin is defined as <5 mm inside the external contour Chest wall includes the ribs and intercoastal musculature but does not include the pectoralis Heart contour begins below the level of where the pulmonary trunk branches into the left and right pulmonary arteries Lungs are to be contoured separately Spinal canal Breasts are to be contoured separately Dose Prescription The total prescribed dose to the PTV is 30 Gy delivered in 5 equal fractions of 6 Gy per day delivered on alternating days over 5 -10 total days. The prescribed dose must cover at least 95% of the PTV.

Inclusion Criteria:

1. Women 50 years or older
2. Low to intermediate grade DCIS or invasive ductal carcinoma
3. Tumor size < 2 cm invasive ductal carcinoma and < 2.5 cm DCIS
4. Well defined lumpectomy cavity on CT
5. Clear surgical ink margins > 2mm for invasive cancer and > 3 mm for DCIS
6. Node negative
7. Unifocal lesion
8. ER and/or PR positive
9. HER-2 negative
10. BRCA negative
11. Lumpectomy cavity must be < 30% of whole breast volume

Exclusion Criteria:

1. Lobular histology
2. Angiolymphatic invasion
3. Multiple foci of disease
4. Lymphovascular invasion
5. Active lupus or sarcoid
6. Distant metastases
7. Non-epithelial malignancies
8. Synchronous contralateral breast cancer
9. Grade 2 or higher oncoplastic surgery
10. Ipsilateral pacemaker
11. Ipsilateral breast implant
12. Neoadjuvant chemotherapy
13. Prior ipsilateral breast cancer or thoracic radiation
14. Poor breast integrity
15. Paget's Disease of the nipple
16. Pregnant patients
17. Severe cardiac, pulmonary, or liver diseases

SBRT for treatment of the surgical cavity following partial mastectomy for women with early-stage breast cancer will be studied.