• Female

• Age \>/= 45 years

• Unicentric pathological stage I (pT1 or T2 pN0\_M0) invasive ductal breast cancer or DCIS measuring \<3 cm in longest diameter on pathology and/or mammogram that is histologically confirmed. If T2, the tumor must be less than 3cm in longest diameter. Note: Women ≥ 70 years or older with T1 invasive ductal carcinoma who are estrogen- receptor positive (ER+) with clinically negative axillary nodes, and do not undergo surgical lymph node mapping or dissection (i.e., if tumor deposit is 0.2 mm or less, regardless of whether the deposit is detected by immunohistochemistry or hematoxylin and eosin staining) will also be eligible. Patients age 50 or older who are clinically node-negative by physical exam and ultrasound and have a primary tumor that is \<=2 cm and hormone-receptor positive, are also eligible.

• Histologically negative tumor margin or no tumor in a re-excision specimen on final shaved specimen.

• ECOG Performance Status of 0 or 1.

• Negative serum pregnancy test within 14 days prior to study treatment if a woman has child-bearing potential. Subjects of child bearing potential are those who have not been surgically sterilized or have not been free from menses for \> 1 year

• Written informed consent obtained from subject and ability for subject or comply with the requirements of the study

• Female subjects of childbearing potential should be willing to use 2 methods or birth control or be surgically sterile or abstain from heterosexual activity for the duration of study participation. Should a woman become pregnant while participating on study, she should inform the treating physician immediately.

• Post-NAC cohort patients: clinical T1-T2 (less than or equal to 5cm in longest diameter on available imaging) and clinical N0-N1 (on exam and imaging) with pathological ypT0, ypTis, or ypT1 (\<=2cm) and ypN0 (any regimen of neoadjuvant chemotherapy agents is allowed)

⁃ OR

• Oncotype RS score of 26 or higher

⁃ OR

• PAM50 ROR scored as HIGH

⁃ OR

• Presence or LVI (focal, limited or not otherwise specified) in the lumpectomy specimen

⁃ OR

• Age 40-49 years (ALL OF THE FOLLOWING MUST BE TRUE : 1) no history of prior benign breast biopsies, 2) no concomitant or prior atypia in either breast, 3) no concomitant or prior LCIS in either breast, 4) no family history of breast cancer in first degree relatives)

⁃ OR

• Invasive lobular carcinoma

⁃ OR

• Surgical margins less than 1 mm to invasive or in-situ disease.