GCC 1926- Phase Ib Dose Escalation of Single-Fraction Preoperative Stereotactic Partial-Breast Irradiation for Early-Stage Breast Cancer

Inclusion Criteria: * Patients must sign consent for study participation. * Patients must be female with a diagnosis of invasive ductal carcinoma. Lobular histologies will not be included, because of difficulty in defining the extent of disease with imaging. * Patients must be deemed appropriate candidates for breast-conserving therapy (i.e., not pregnant, never had RT to the treated breast, breast size would allow adequate cosmesis after volume loss from partial mastectomy). * Tumor must not involve the overlying skin or underlying chest wall, based on imaging evaluation and/or clinical exam. * Greatest tumor dimension is \<3 cm based on US. MR imaging measurements can be included only if performed BEFORE the biopsy (postbiopsy measurements can be larger as a result of hematoma formation or increased edema). * Tumor must be unifocal. * Patients must be \> 45 years old. * The tumor must be visible on a CT scan. * Patients must undergo MR imaging for work-up to aid in tumor delineation and to rule out additional foci of disease. If additional foci of disease are found to be present, then these must be biopsied with negative results to proceed with treatment. * The tumor must be clinically and radiographically N0 (node negative). If a suspicious node is visualized, it must be biopsied with negative results. * Patients must be estrogen-receptor positive. * Patients must be HER2neu negative. * Patients must weigh \<150 kg (330 lb), which is the limit of the imaging loader. * Patients must be \<6'6" in height, again because of instrumentation limitations. * Patients must be able to lie prone for treatment. * Patients must have no lymphovascular invasion on biopsy. * Patients may be taking hormonal therapy prior to initiation of treatment. This will be documented. Exclusion Criteria: * Multicentric disease. * Prior RT to the involved breast. * Inability to fit into the immobilization breast cup because of breast size or other anatomic reasons. * Inability to obtain an adequate seal when using the immobilization breast cup. * Male sex. * Breast implants. * Patient cannot comfortably lie in the prone position (i.e., physical disability). * Patients who are planned to be treated surgically with a mastectomy. * Tumor \<5 mm from the skin surface or chest wall on clinical exam and/or radiographic imaging. * Tumor size \>3 cm. * Patients with psychiatric or addictive disorders that would preclude obtaining informed consent or study completion. * Patients who are pregnant or lactating (because of potential RT exposure to the fetus and unknown effects of RT in lactating women). * Patient unable to undergo MR imaging (e.g., because of known contrast reaction or concerns about contrast and existing health status). * Lymphovascular invasion on original biopsy. * Tumor histologies other than invasive ductal carcinoma (including invasive lobular carcinoma). * ER-negative tumors. * HER2neu-positive tumors. * Pure ductal carcinoma in situ (no invasive component). * Weight \>330 lb * Prior ipsilateral breast cancer. * Diffuse calcifications on mammogram (BIRADS 3, 4, 5).