Inclusion Criteria:

          -  Histologically confirmed diagnosis of invasive or non-invasive breast cancer.

          -  Invasive ductal, medullary, papillary, colloid (mucinous) or tubular histologies.

          -  AJCC T1 or T2; N0 or N1mic; Stage IA-IIA breast cancer or AJCC TIS (Stage 0) ductal
             carcinoma in situ without invasion

          -  Gross disease must be unifocal with pathologic (invasive and/or DCIS) tumor size 3 cm
             or less. (Patients with microscopic multifocality are eligible as long as total
             pathological size is 3 cm or less).

          -  Estrogen and/or progesterone receptor positive invasive breast cancer. DCIS stage 0
             does not require receptor testing.

          -  No evidence of distant metastatic disease as documented by history and physical
             examination (radiographic staging only to be performed as indicated by symptoms or
             physical findings.)

          -  Patients must have an ECOG Performance Status of 0, 1 or 2

          -  Age ≥ 50.

          -  Patients must be able to provide informed consent.

          -  Patients must have undergone breast-conserving surgery

          -  All tumors (invasive and non-invasive disease) must be excised with a minimum margin
             width of ≥ 2 mm. Re-excision of surgical margins is permitted. Focally close (<2 mm)
             or positive (tumor cells at the inked edge of the specimen) margins determined to be
             at an anatomic boundary of resection by the surgeon, such as posterior fascia for
             posterior margins or skin for anterior margins, are also acceptable.

          -  Patients with invasive breast cancer must be node-negative (N0) or have only
             microscopic disease (≤2mm) in the nodes (N1mi). Patients with Stage IA - IIA are
             required to have axillary staging but it will not be done for patients with Stage 0
             DCIS. Options for axillary staging include:

               1. Negative sentinel lymph node biopsy (SLNB)

               2. Level I-II axillary lymph node dissection (ALND) (6 or more nodes removed).

               3. Positive SLNB followed by completion ALND (6 or more nodes removed).

          -  Patients presenting with abnormal microcalcifications on a screening mammogram must
             have radiographically confirmed excision of the suspicious microcalcifications, either
             by specimen radiograph or post-biopsy mammograms.

          -  The patient must be enrolled on the study within 50 days following the last surgery
             for breast cancer (lumpectomy, re-excision of margins, or axillary staging procedure).

          -  The target lumpectomy cavity must be clearly delineated and the target lumpectomy
             cavity/whole breast reference volume must be ≤ 30% based on the
             postoperative/pre-enrollment CT scan.

          -  Patients must have bilateral mammogram and/or breast MRI within 3 months of diagnosis
             of their breast cancer.

          -  CBC/differential obtained within 3 months prior to registration on study, with
             adequate bone marrow function defined as follows: Absolute neutrophil count (ANC) ≥
             1,800 cells/mm3; Platelets ≥ 75,000 cells/mm3; Hemoglobin ≥ 8.0 g/dl (Note: The use of
             transfusion or other intervention to achieve Hgb ≥ 8.0 g/dl is acceptable).

          -  Patients with synchronous bilateral breast cancers who will be treated with
             radiotherapy to each breast are eligible, provided such treatment can be performed in
             a manner that avoids overlap between treatment fields. Both sides may be treated with
             APBI if the pathologic eligibility criteria are met for both tumors, or only one side
             may be treated with APBI if the criteria are met for only one tumor.

          -  Patients with a history of prior breast cancer in the opposite breast are eligible as
             long as treatment can be performed without overlapping any prior RT fields.

          -  Patients with a history of prior breast cancer in the ipsilateral breast treated with
             lumpectomy alone (no RT) are eligible as long as the other entry criteria for this
             study are met.

          -  Patients with a history of non-breast malignancies are eligible as long as they have
             not received prior radiotherapy to the thoracic region, and have a greater than 2 year
             interval without evidence of recurrence.

          -  Women of childbearing potential must be non-pregnant and non-lactating and willing to
             exercise an effective form of birth control during radiation therapy (e.g. oral
             contraceptive, IUD, condoms or other barrier methods etc.). Hysterectomy or menopause
             must be clinically documented.

          -  Patient must provide study-specific informed consent prior to study entry.

        Exclusion Criteria

          -  Male breast cancer

          -  T2 (>3cm), T3, T4, Node positive (other than N1mi), or M1 disease

          -  Lobular or mixed ductal and lobular histology.

          -  Multifocal primary tumor.

          -  Clear delineation of the extent of the lumpectomy cavity is not possible. Prior or
             simultaneous malignancies within the past two years (other than carcinoma in situ of
             the cervix, CIS of the colon, melanoma in situ, thyroid cancer, and basal cell or
             squamous cell carcinoma of the skin).

          -  Any non-axillary sentinel node(s) positive. (Note that intramammary nodes are staged
             as axillary nodes).

          -  Patients who have had a positive SLNB but decline completion ALND are not eligible.

          -  Patients treated with neoadjuvant chemotherapy are not eligible.

          -  Palpable or radiographically suspicious ipsilateral or contralateral axillary,
             supraclavicular, infraclavicular, or internal mammary nodes, unless there is
             histological confirmation that these nodes are negative for tumor.

          -  Suspicious microcalcifications, densities, or palpable abnormalities (in the
             inpsilateral or contralateral breast) unless these were biopsied and found to be
             benign.

          -  Proven multicentric carcinoma (invasive cancer or DCIS) in more than one quadrant or
             two or more breast cancers not resectable through a single lumpectomy incision.

          -  Paget's disease of the nipple.

          -  Surgical margins that cannot be microscopically assessed or are positive at
             pathological evaluation. A focally positive margin determined to be at an anatomic
             boundary of resection by the surgeon, such as posterior fascia for posterior margins
             and skin for anterior margins, is also acceptable. If surgical margins are rendered
             free of disease by re-excision, the patient is eligible.

          -  Breast implants. (Patients who have implants removed are eligible).

          -  Prior ipsilateral breast or thoracic radiation for any condition.

          -  Collagen vascular disease, specifically dermatomyositis with a CPK level above normal
             or with an active skin rash, systemic lupus erythematosis, or scleroderma.

          -  Pregnant women, women planning to become pregnant and women that are nursing.

          -  Psychiatric or addictive disorders or other conditions that, in the opinion of the
             investigator, would preclude the patient from meeting the study requirements.

          -  Actively being treated on any other therapeutic research study.