This trial will investigate a novel 3-fraction radiation regimen for participants undergoing breast-conserving therapy (BCT) for early breast cancer that will: 1) significantly reduce the duration of treatment and can be completed in one-week (5 working days) and 2) MRI-guided radiotherapy (MRIdian) would limit the volume of normal tissue radiated and therefore resultant toxicity. The hypothesis is that 3-fraction radiation therapy can be delivered safely without compromising the therapeutic ratio. Participants can expect to be on study for follow up up to 5 years.

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.