The Purpose of this study is to evaluate the feasibility of using image guidance and four-dimensional computed tomography to reduce the volume of non target breast tissue that is treated with accelerated partial breast irradiation via the external beam technique.

This is a pilot study enrolling about 30 patients to evaluate the feasibility of using 4D CT and daily image guidance in Accelerated Partial Breast Irradiation (APBI) to decrease set-up error and individualized planning target volumes to limit the non-target breast tissue dose. The two specific breasts constraints which shall be decreased are the V50 (percent volume of breast receiving 50% of the prescribed dose) and the V100 (percent volume of breast receiving 100% of the prescribed dose). The goal will be to decrease these to 45% and 23.5% respectively, which is down 33% from the NSABP B39 standard of 60% and 35%.

Inclusion Criteria:

• The patient must consent to be in the study and must have a signed an approved consent form conforming with institutional guidelines.
• Patient must be > 50 years old.
• The patient should have a life expectancy of at least two years with a karnofsky performance status > 70.
• The patient must have stage 0 or I breast cancer.
• On histological examination, the tumor must be DCIS or invasive adenocarcinoma of the breast.
• Surgical treatment of the breast must have been lumpectomy. The margins of the resected specimen must be histologically free of tumor (>2mm, DCIS and invasive). Re-excision of surgical margins is permitted.
• Gross disease must be unifocal with pathologic (invasive and/or DCIS) tumor size 2 cm or less. (Patients with microscopic multifocality are eligible as long as total pathologic tumor size is 2 cm or less.)
• Patients with invasive breast cancer are required to have axillary staging which can include sentinel node biopsy alone (if negative), sentinel node biopsy followed by axillary dissection or sampling with a minimum total of 6 axillary nodes or axillary dissection alone (with a minimum of 6 axillary nodes). Axillary staging is NOT required for patients with DCIS.
• The patient must have simulation within 8 weeks/56 days of the final surgery for their breast cancer (lumpectomy, re-excision of margins, or axillary staging procedure).
• Patients with a history of non-ipsilateral breast malignancies are eligible if they have been disease-free for 2 or more years prior to enrollment. Patients with the following cancers are eligible even if diagnosed and treated within the past 2 years: carcinoma in situ of the cervix, colon, melanoma in situ, and basal cell and squamous cell carcinoma of the skin.
• Chemotherapy is permitted if planned for ≥ 2 weeks after radiation.
• Urine pregnancy test must be performed and be negative on all women younger than 60 who have not had a tubal ligation, oophorectomy, or hysterectomy.
• Separate incisions for the lumpectomy and sentinel node biopsy should be present. Use of only one incision will typically result in a contiguous cavity with the tumor bed and the sentinel node sampling, and inability of the radiation oncologist to delineate the tumor bed from the sentinel node bed.
• The patient must have a cavity which is able to be targeted with external beam APBI, either through surgical clip placement, or CVS 3 or higher. The cavity to whole breast ratio must be 30% or less.

Exclusion Criteria:

• Men are not eligible for this study as men are not breast conservation candidates.
• T0, T2 (> 2.0 cm), T3, node positive, stage III or IV breast cancer.
• Any positive axillary nodes.
• Palpable or radiographically suspicious ipsilateral or contralateral axillary, supraclavicular, infraclavicular or internal mammary nodes, unless biopsy proven to be negative for tumor.
• Suspicious microcalcifications, densities or palpable abnormalities in either breast unless biopsy proven to be benign.
• Non-epithelial breast malignancies such as sarcoma or lymphoma.
• Proven multicentric carcinoma in more than one quadrant or separated by more than 2 centimeters.
• Paget's disease of the nipple.
• History of invasive breast cancer or DCIS in the same breast.
• Surgical margins that cannot be microscopically assessed or are less then 2 mm.
• Collagen vascular disease, specifically dermatomyositis with a CPK level above normal or with an active skin rash, systemic lupus erythematosis or scleroderma.
• Pregnancy or lactation at the time of proposed radiation. 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 treating physician, would preclude the patient from meeting the study requirements.
• Patients with coexisting medical conditions in whom life expectancy is < 2 years.
• Patients with skin involvement, regardless of tumor size.

Through treatment completion, and up to 3 years follow up.