Partial Breast Radiation Therapy in Treating Women Undergoing Breast Conservation Therapy for Early-Stage Breast Cancer

DISEASE CHARACTERISTICS: * Women who have chosen to undergo breast conservation therapy, including lumpectomy and breast irradiation, at the University of Pennsylvania for any invasive mammary carcinoma or intraductal breast cancer * Stage I-II invasive or intraductal breast cancer * Unifocal tumor ≤ 3.0 cm in size * Patients with microscopic multifocality are eligible provided total pathologic tumor size is ≤ 3 cm * No proven multicentric carcinoma in more than 1 quadrant or separated by 4 or more centimeters * Pre- or post-biopsy ipsilateral\* breast MRI negative for multicentric disease (i.e., areas of cancer that cannot be removed in a single excision specimen) or other suspicious findings NOTE: \*Patients with synchronous bilateral breast cancers who will be treated with radiotherapy to each breast are eligible, provided such treatment can be performed in a manner that avoids overlap between treatment fields. Both sides may be treated with partial breast irradiation (PBI) if the pathologic eligibility criteria are met for both tumors, or only one side may be treated with PBI if the criteria are met for only one tumor. * Negative margins of excision (≥ 2 mm) OR no tumor seen in a re-excision specimen * No extensive intraductal component present * Negative sentinel lymph node (SLN) or axillary lymph node dissection OR fewer than 4 positive nodes on adequate axillary lymph node dissection (i.e., 10 or more lymph nodes removed) * If a SLN is positi…