• To evaluate the rate and severity of cutaneous toxicity [ Time Frame: 10 years ] [ Designated as safety issue: Yes ]
• To evaluate the risk of breast fibrosis and fat necrosis [ Time Frame: 10 years ] [ Designated as safety issue: No ]
• To evaluate cosmetic outcome [ Time Frame: 10 years ] [ Designated as safety issue: No ]
• To evaluate patient satisfaction [ Time Frame: 10 years ] [ Designated as safety issue: No ]
• To evaluate local and distant control rates [ Time Frame: 10 years ] [ Designated as safety issue: No ]

The purpose of this study is to determine the feasibility of partial breast irradiation in participants with early invasive or non-invasive breast cancer.

• In this study, radiotherapy is given only to a portion of the breast around the tumor bed using external radiation treatments. This approach is called "partial breast irradiation".
• Participants will undergo a planning session for the radiation treatments. This planning will be done by CT scan days or weeks before the start of treatment. This planning process is the same as that used to plan conventional radiation therapy treatments.
• Radiation therapy will begin 4-12 weeks after the last surgery of the breast in individuals not receiving chemotherapy first. For individuals receiving chemotherapy before radiation therapy, radiation therapy will start 2-6 weeks after ending chemotherapy.
• We will be studying three levels of radiation doses to see which is best. The dose the participant receives will depend upon when they are enrolled on the trial.
• Participants will receive radiation treatment twice each day for 4 or 5 treatment days, with an overall treatment time of one week.

• Experimental: Study Participants 1-100
• Experimental: Study Participants 101-200
• Experimental: Study Participants 201-400

Inclusion Criteria:

• Histologically confirmed unicentric Stage I Invasive Ductal breast cancer. Histologically negative tumor margin 2mm or more from any inked edges, or no tumor in re-excision specimen or final shaved specimen.
• Patient may have been treated with adjuvant chemotherapy, or be on adjuvant hormonal therapy or begin hormonal therapy following XRT
• 18 years of age or older
• ECOG Performance Status 0
• Required laboratory data as outlined in the protocol

Exclusion Criteria:

• Multicentric IDC of the breast defined as discontiguous tumors separated by at least 5cm of uninvolved tissue
• Multifocal IDC of the breast, defined as discontiguous discrete foci of invasive carcinoma, separated by uninvolved intervening tissue, but within an overall span of 5cm, or within the same breast quadrant or subareolar central region
• Tumor > 2.0cm, nodal involvement, or metastatic involvement
• Histological evidence of: lymphovascular invasion; blood vessel invasion; extensive intraductal component; invasive lobular carcinoma and infiltrating carcinoma of mixed ductal and lobular type; DCIS with microinvasion and DCIS suspicious for microinvasion; infiltrating micropapillary carcinoma
• Known mutation carrier, including BRCA1 and BRCA2
• History of cosmetic or reconstructive breast surgery
• Psychiatric illness which would prevent the patient from giving informed consent
• Medical conditions such as uncontrolled infection, uncontrolled diabetes mellitus or connective tissue disease
• Participants with a "currently active" second malignancy other than non-melanoma skin cancers
• Patients with diffuse (> 1 quadrant or >5cm) suspicious microcalcifications
• Women who are pregnant

• Dana-Farber Cancer Institute
• Brigham and Women's Hospital
• Beth Israel Deaconess Medical Center
• Boston Medical Center