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 creatine phosphokinase (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