DISEASE CHARACTERISTICS: Histologically confirmed nonlobular invasive breast cancer of one of the following histologies: Ductal Papillary Medullary Colloid (mucinous) Tubular Stage I/II (T1-2 N0-1 M0) tumor no greater than 3 cm Prior tylectomy and axillary lymph node dissection required No more than 3 positive axillary lymph nodes and no extracapsular extension At least 6 lymph nodes sampled OR Negative sentinal node Surgical clips in place delineating margins of tylectomy cavity Negative, or close but negative, inked histologic margins of tylectomy or re-excision specimen Negative margins defined as no tumor at least 2 mm away from the inked margins Close but negative margins defined as tumor within 2 mm of inked margin Positive margins defined as invasive or noninvasive tumor at inked resection margins Negative post-tylectomy or post-reexcision mammogram required if disease presented with malignancy-associated microcalcifications No remaining suspicious microcalcifications If catheters are placed at time of reexcision, the crieria may not be feasible but a post brachytherapy mammogram should be obtained as soon as possible No extensive intraductal carcinoma by the Harvard definition, i.e.: More than 25% of tumor is ductal carcinoma in situ (DCIS) and there is DCIS in adjacent breast tissue OR Intraductal carcinoma with microinvasion No proven multicentric carcinoma, i.e.: Tumor in different quadrants of the breast OR Tumor separated by at least 4 cm The following exclude: Skin or chest wall invasion Matted or fixed axillary adenopathy Metastatic internal mammary or supraclavicular nodes Paget's disease of the nipple Previously treated contralateral breast carcinoma or synchronous bilateral breast carcinoma Hormone receptor status: Not specified

PATIENT CHARACTERISTICS: Age: Any age Sex: Female Menopausal status: Not specified Performance status: Not specified Hematopoietic: Not specified Hepatic: Not specified Renal: Not specified Other: No collagen vascular disease with a c-reactive protein level above normal or with an active skin rash (e.g., systemic lupus erythematosus, scleroderma, or dermatomyositis) No coexisting medical condition that reduces life expectancy to less than 2 years No second malignancy within 5 years except nonmelanomatous skin cancer Not pregnant or nursing Negative pregnancy test

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior chemotherapy for breast cancer Planned chemotherapy may begin at least 2 weeks after removal of brachytherapy catheters Endocrine therapy: Concurrent tamoxifen permitted Radiotherapy: No prior radiotherapy for breast cancer Surgery: See Disease Characteristics