Inclusion Criteria:

- All patients aged 45 years or older with operable invasive breast cancer, T1 and T2
(< 3.5 cm), N0-1, M0, confirmed by cytological or histological examination, who are
suitable for breast conserving surgery are eligible as long as they have had an
ipsilateral diagnostic mammogram within 12 months of enrollment.

- Those with previously diagnosed and treated contralateral breast cancer may be
entered.

- It is recommended that patients meet an ECOG performance status of 0-3, however,
grade 4 patients can be treated at the discretion of the participating center.

- Individual centers may wish to restrict entry to a more exactly defined subset of
patients in which case only patients with these characteristics may be entered by the
particular center. For example, centers may decide at outset to recruit only women
over the age of 50 or possibly only postmenopausal women. Before entering any patient
into the trial, the local investigator should confirm that the patient would be
available for regular follow-up for at least 5 years.

Exclusion Criteria:

- Age < 45 years

- Axillary lymph node positive breast cancer

- Invasive lobular cancer

- Tumor size > 3.5 cm

- Extensive Intraductal Component (EIC= > 25% of the lumpectomy specimen involved with
ductal carcinoma in situ, DCIS) as assessed on surgical pathologic lumpectomy
specimen

- Multicentric cancer in the same breast as diagnosed by clinical examination,
mammography, ultrasound. MRI or pathologic assessment, not amenable to excision with
negative margins with a single lumpectomy.

- Inability to assess pathologic margin status

- Bilateral breast cancer at the time of diagnosis.

- Ipsilateral breast had a previous cancer and/or prior in-field radiation.

- Patients known to have BRCA1/2 gene mutations (testing for gene mutations is not
required).

- Patients undergoing primary systemic treatment (hormones or chemotherapy) as initial
treatment with neoadjuvant intent of reducing tumor size.

- No more than 30 days can have elapsed between the last breast cancer surgery (not
axillary) and entry into the trial for patients in the post-pathology stratification.

- Previous history of malignant disease does not preclude entry if the expectation of
relapse-free survival at 10 years is 75% or greater

- Any factor included as exclusion criteria in the participating center's Treatment
Policy Statement.