Partial Breast Irradiation in a Low-risk Population Screened With MRI

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