Inclusion Criteria:

* Suitable for breast conserving surgery
* T1 and T2 (\< 3.5 cm), N0, M0

Exclusion Criteria:

* Axillary lymph node positive breast cancer
* Tumor size \> 3.5 cm
* Extensive intraductal component (EIC \>= 25% of the lumpectomy specimen involved with ductal carcinoma in situ), as assessed on surgical pathologic lumpectomy specimen
* Multicentric cancer in the same breast as diagnosed by clinical examination, mammography, ultrasound; magnetic resonance imaging (MRI) or pathologic assessment, not amenable to excision with negative margins with a single lumpectomy
* Inability to assess pathologic margin status
* Synchronous 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 reducing tumor size
* Previous history of malignant disease does not preclude entry if the expectation of relapse-free survival at 10 years or greater
* Any factor included as exclusion criteria in the participating center's treatment policy statement
* Additional exclusion criteria for University of California San Francisco (UCSF) (as laid out in the Treatment Policy):

* Patients under the age of 50
* Estrogen receptor negative (as defined in Treatment Policy under "Pathology")
* Human epidermal growth factor receptor 2 (HER2) positive (as defined in Treatment Policy under "HER2")
* Lymphovascular invasion
* High grade
* Tumors \> 3 cm
* Node positive patients
* Prior chemotherapy or hormone therapy