Inclusion Criteria:

- Stage 0 or I breast carcinoma

- 40 years or older

- Post menopausal

- Lumpectomy with clear margins (>2mm)

- Patients with invasive disease must have undergone an axillary staging by axillary
dissection (with a minimum of 6 axillary nodes) or sentinal node biopsy

- Gross disease must be unifocal with pathological tumor size 2cm or less

- Target lumpectomy cavity must be clearly marked with surgical clips (the target
lumpectomy cavity/whole breast reference volume must be <=30% based on the
postoperative imaging)

- Bilateral MRI of the breasts within previous 6 months of radiation therapy (RT)
planning

- If patient is eligible based on postoperative imaging, partial breast irradiation
(PBI) is judged to be technically deliverable

- Patients with a history of malignancies are eligible if they have stable disease as
determined by their attending oncologist

- Patient must have signed the consent form

Exclusion Criteria:

- Men are not eligible

- Patients with Stage II, II, or IV breast cancer

- Pre- or peri-menopausal patients

- Patients with positive lymph nodes

- Patients with suspicious microcalcifications, densities, or palpable abnormalities
unless biopsied and found to be benign

- Patients with multifocal, multicentric, or bilateral breast cancer

- Surgical margins that cannot be microscopically assessed or are positive at
pathological evaluation

- Patients with a history of breast cancer

- Clear delineation of the extent of the target lumpectomy cavity is not possible

- Breast implants (patients who have had implants removed are eligible)

- Prior breast or thoracic RT for any condition