A Phase II Study of Real-Time MRI-Guided 3-Fraction Accelerated Partial Breast Irradiation in Early Breast Cancer (MAPBI)

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.