This study will use an adjuvant radiation therapy regimen called APBI (accelerated partial breast irradiation). APBI is a treatment option available to women considered to have an early stage, low-risk breast cancer. The standard external beam-based APBI treatment delivers treatments every other day for five treatments, over ten calendar days. The APBI in this study is modified and will deliver five once daily treatments over consecutive treatment days, with a slightly lower dose of radiation each day as compared to the standard external beam-based APBI treatment.

This is a prospective observational cohort study in breast cancer participants. The purpose of this study is to report physician-assessed cosmetic outcomes.

Inclusion Criteria:

• Ability to provide informed written consent.
• Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 3 or a Karnofsky performance score of ≥ 30%.
• Willingness to return to Sanford Cancer Center for follow-up.
• Radiation therapy is planned as part of routine care.
• On histological examination, the tumor must be either ductal carcinoma in-situ (DCIS) or an invasive ductal carcinoma of the breast.
• Patients must be ≥ 50 years old.
• Presence of unifocal tumor.
• Surgical treatment of the breast must have been a lumpectomy. Placement of surgical clips to delineate the lumpectomy bed is highly recommended, though not required.
• The final margins of the resected specimen must be histologically free of tumor (≥ 2 mm for invasive adenocarcinoma and ≥ 3 mm for DCIS). Re-excision of surgical margins is permitted. Gross disease must be unifocal with a pathologic invasive tumor size tumor size ≤ 2.0 cm or pathologic DCIS size ≤ 2.5 cm.
• Patients with invasive breast cancer are required to have axillary staging consisting of a sentinel lymph node biopsy. The sentinel lymph node(s) must be negative for regionally metastatic disease, both on routine H&E evaluation and immunohistochemical staining. Axillary staging is not required for patients with DCIS.
• Absence of lymphovascular space invasion.
• The tumor must be positive for either the presence of the estrogen or progesterone receptor, and either 0 or +1 for the HER-2/neu receptor on immunohistochemical staining.
• The target lumpectomy cavity must be clearly delineated and the target lumpectomy cavity to the whole breast reference volume ratio must be ≤ 30% based on the CT-simulation scan.
• Life expectancy of 5 years or longer excluding breast cancer diagnosis(co-morbid conditions taken into considerations by the PI)
• Women of child-bearing potential must have a negative pregnancy test before their CT simulation, and must agree to use an effective method of birth control during their radiation therapy treatment course.

Exclusion Criteria:

• Pregnant women, or women of childbearing potential who are unwilling to employ adequate contraception.
• Nuclear grade 3 DCIS or Nottingham grade 3 invasive ductal carcinoma.
• Invasive lobular carcinoma, any grade.
• Male sex.
• Any treatment with radiation therapy, and/or hormonal therapy administered for the currently diagnosed breast cancer prior to study enrollment
• Prior breast or thoracic radiation treatment for any reason.
• History of prior invasive breast cancer or DCIS. (Patients with a history of LCIS treated by surgery alone are eligible.)
• Paget's disease of the nipple.
• 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.
• Patients with a collagen vascular disease; specifically, SLE or scleroderma.
• Breast implants. (Patients who have had implants removed are eligible.)
• Psychiatric or addictive disorders or other conditions that, in the opinion of the investigator, would preclude the patient from meeting the study requirements.
• Women with a known or suspected predisposition to developing breast cancer (i.e., BRCA1/2 mutations, p53 mutation, etc...)
• Women who are pregnant, or women of child-bearing potential who refuse to use an effective method of birth control

Women

No plan to share IPD with other researchers