Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed ductal carcinoma in situ (DCIS*) or invasive* adenocarcinoma
of the breast

- Stage 0, I, or II disease

- Stage II tumors must be ≤ 3 cm

- Gross disease must be unifocal

- Microscopic multifocality allowed provided total pathological tumor size is
≤ 3 cm

- No proven multicentric carcinoma in more than 1 quadrant or separated by ≥ 4 cm

- No non-epithelial breast malignancies (e.g., sarcoma or lymphoma) NOTE:
*Patients who are 50 years of age and over with DCIS regardless of hormone
receptor status AND patients with invasive breast cancer meeting all of the
following criteria: ≥ 50 years of age, node-negative, and hormone-receptor
positive status will not be enrolled in study after 12/30/2006

- Prior axillary staging required for patients with invasive breast cancer, including 1
of the following:

- Sentinel node biopsy alone (if sentinel node is negative)

- Sentinel node biopsy followed by axillary dissection or sampling with ≥ 6
axillary nodes (if sentinel node is positive)

- Axillary dissection alone with ≥ 6 axillary nodes

- No more than 3 positive axillary nodes

- No axillary nodes with definite evidence of microscopic or macroscopic
extracapsular extension

- No positive non-axillary sentinel nodes (intramammary nodes are staged as
axillary nodes)

- No 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

- Must have undergone lumpectomy

- Resected margins histologically free of tumor

- Re-excision of surgical margins allowed

- Target lumpectomy cavity clearly delineated AND target lumpectomy/whole breast
reference volume ≤ 30% based on postoperative pre-randomization CT scan

- Final surgery (i.e., lumpectomy, re-excision of margins, or axillary staging
procedure) within the past 42 days

- No suspicious microcalcifications, densities, or palpable abnormalities in the
ipsilateral or contralateral breast unless biopsied and found to be benign

- No Paget's disease of the nipple

- No history of invasive breast cancer or DCIS

- Prior lobular carcinoma in situ treated by surgery alone allowed

- No synchronous bilateral invasive or non-invasive breast cancer

- Partial breast irradiation deemed technically deliverable by radiation oncologist at
a credentialed facility

- Must have undergone a history and physical exam within the past 4 months AND a
bilateral mammogram within the past 6 months

- Hormone receptor status:

- Estrogen receptor (ER) status known

- Progesterone status known if ER analysis is negative

- Marginal or borderline results are considered positive

PATIENT CHARACTERISTICS:

Age

- 18 and over

Sex

- Female

Menopausal status

- Premenopausal or postmenopausal

Performance status

- Not specified

Life expectancy

- At least 10 years, excluding diagnosis of breast cancer

Hematopoietic

- Not specified

Hepatic

- Not specified

Renal

- Not specified

Other

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective non-hormonal contraception

- No other malignancy within the past 5 years except previously treated carcinoma in
situ of the cervix or colon, melanoma in situ, or basal cell or squamous cell skin
cancer

- Deemed to be at low risk for recurrence

- No collagen vascular disease (e.g., systemic lupus erythematosus or scleroderma),
specifically dermatomyositis with a CPK level above normal, or active skin rash

- No psychiatric or addictive disorder that would preclude study therapy

PRIOR CONCURRENT THERAPY:

Biologic therapy

- No prior biologic therapy for this malignancy

Chemotherapy

- No prior chemotherapy for this malignancy

- No concurrent chemotherapy during study radiotherapy

Endocrine therapy

- No prior hormonal therapy for this malignancy unless total duration of hormonal
therapy was no more than 28 days before randomization

- Concurrent hormonal therapy allowed provided it is not administered during
chemotherapy

- No concurrent raloxifene, tamoxifen, or other selective estrogen receptor modulating
drugs

- No concurrent hormone replacement therapy

- No concurrent Femring^®

Radiotherapy

- No prior radiotherapy for this malignancy

- No prior breast or thoracic radiotherapy

- No concurrent brachytherapy boosts

- No concurrent intensity modulated radiotherapy

- No concurrent regional nodal irradiation

Surgery

- See Disease Characteristics

- No prior breast implants

- Patients who have had implants removed are eligible

Other

- No other concurrent anticancer therapy