Inclusion Criteria

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