Inclusion Criteria:
For all participants
* Participants should have no contraindications to undergo MRI scan as part of radiotherapy planning and treatment.
* Lumpectomy cavity must be clearly visible on CT and MRI scan at radiotherapy simulation.
* Pregnancy test negative in women of child bearing potential (WOCBP).
* The participant must consent to be in the study and must have signed an approved consent form conforming with institutional guidelines.
* Participants with a history of non-breast malignancies are eligible if they have been disease-free for 5 or more years prior and are deemed by their physician to be at low risk for recurrence. Participants with the following cancers are eligible if diagnosed and treated within the past 5 years: carcinoma in situ of the cervix, carcinoma in situ of the colon, melanoma in situ, and basal cell and squamous cell carcinoma of the skin.
For participants with Invasive Carcinoma
* Suitable:
* Age: \>=50 years
* Margins: Negative by at least 2 mm
* T Stage: Tis or T1
* Cautionary:
* Age: 40-49 years
* Margins: Negative by at least 2 mm
* T Stage: Tis or T1 OR
* Age: \>=50 years IF participant has at least 1 of the pathologic factors below and does not have any "unsuitable" factors (below)
* Pathologic Factors:
* Size 2.1-3.0 cm (size of the invasive component)
* T2
* Close margins (\<2 mm)
* Limited/focal Lymphovascular Space Invasion (LVSI)
* ER (-)
* Clinically unifocal with total size 2.1-3.0 cm (Microscopic multifocality allowed, provided the lesion is clinically unifocal (a single discrete lesion by physical examination and ultrasonography/mammography) and the total lesion size (including foci of multifocality and intervening normal breast parenchyma) falls between 2.1 and 3.0 cm).
* Invasive lobular histology
* Extensive Intraductal Component (EIC) \<=3 cm
For participants with DCIS
* Suitable Criteria, DCIS allowed if all of the following are met:
* Screen-detected
* Low to intermediate nuclear grade
* Size \<=2.5 cm
* Resected with margins negative at \>=3mm OR
* Cautionary Criteria:
* Pure DCIS \<=3 cm if "suitable" criteria not fully met
Exclusion Criteria (unsuitable criteria) :
* Men are not eligible for this study.
* BRCA1/2 mutation positivity.
* Age \< 40 years (American Society for Radiation Oncology (ASTRO) Unsuitable Criteria).
* Positive resection margins on post operative pathology(ASTRO Unsuitable Criteria).
* Palpable or radiographically suspicious ipsilateral or contralateral axillary, supraclavicular, infraclavicular, or internal mammary nodes, unless there is histologic confirmation that these nodes are negative for tumor.
* Suspicious micro calcifications, densities, or palpable abnormalities (in the ipsilateral or contralateral breast) unless biopsied and found to be benign.
* Non-epithelial breast malignancies such as sarcoma or lymphoma.
* Proven multicentric carcinoma (invasive cancer or DCIS) (ASTRO Unsuitable Criteria).
* Pure DCIS \>3 cm in size (ASTRO Unsuitable Criteria).
* Presence of extensive intraductal component \>30mm (ASTRO Unsuitable Criteria).
* Paget's disease of the nipple.
* History of previous invasive breast cancer, DCIS, synchronous bilateral invasive or non-invasive breast cancer. (Participants with a history of LCIS treated by surgery alone are eligible.)
* Surgical margins that cannot be microscopically assessed or are positive at pathologic evaluation.
* Concurrent therapy with any hormonal agents such as raloxifene (Evista®), tamoxifen, Aromatase Inhibitors or other selective estrogen receptor modulators (SERMs), either for osteoporosis or breast cancer prevention or neoadjuvant therapy.
* Breast implants.
* Prior breast or thoracic radiotherapy for any condition or treatment plan that includes regional nodal irradiation.
* Collagen vascular disease, specifically dermatomyositis with a creatinine phosphokinase level above normal or with an active skin rash, systemic lupus erythematosus, or scleroderma.
* Pregnancy or lactation at the time of treatment. Women of reproductive potential must agree to use an effective non-hormonal method of contraception during therapy.
* Psychiatric or addictive disorders or other conditions that, in the opinion of the investigator, would preclude the participant from meeting the study requirements.
