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 positive on hematoxylin and eosin (but not by immunohistochemistry alone), complete axillary lymph node dissection is required * Axillary lymph node staging is not required for patients with ductal carcinoma in situ * No SLN identified in the internal mammary nodes * No node \> 2 cm * No node with extracapsular extension * Surgical clips placed in the operative bed OR ability to visualize operative bed on CT scan of the breast * Target lumpectomy cavity/whole breast reference volume must be ≤ 30% based on the treatment planning CT scan * No diffuse calcifications on diagnostic mammogram * Negative post-biopsy mammogram required if presented with mammographically detected microcalcifications * Hormone receptor status unspecified PATIENT CHARACTERISTICS: * Female * Menopausal status not specified * History of non-breast malignancies allowed provided patients have been disease free for 5 or more years prior to randomization and are deemed by their physician to be at low risk for recurrence * Treated carcinoma in situ of the cervix, carcinoma in situ of the colon, melanoma in situ, or basal cell and squamous cell skin cancer within the past 5 years allowed * Patients must agree to undergo breast MRI * No contraindication to MRI, including a pacemaker or other foreign body * Not pregnant or nursing * No technical impediment to appropriate dosimetry * No personal history of collagen vascular disease PRIOR CONCURRENT THERAPY: * See Disease Characteristics * No prior in-field irradiation * No presence of breast implant * No breast reconstructive surgery prior to study entry * No prior neoadjuvant chemotherapy or hormonal therapy