Radiation Therapy in Treating Women With Early Stage Breast Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed breast cancer, including 1 of the following subtypes: * Ductal carcinoma in situ * Invasive ductal carcinoma * Invasive lobular carcinoma * Medullary carcinoma * Papillary carcinoma * Colloidal (mucinous) carcinoma * Tubular carcinoma * Pathological stage 0-IIIA disease (pTis; pT1-2, N0-N2a, M0) * Tumor size ≤ 5 cm * Breast considered technically satisfactory for radiotherapy * Has undergone lumpectomy or mastectomy and either sentinel node biopsy or axillary dissection (if invasive carcinoma is present) * Negative inked histological margins (i.e., no invasive cells at surgical margin) or confirmed negative re-excision specimen * Unifocal or multifocal (confined to 1 quadrant; tumors \< 4 cm apart) disease with 1 or 2 foci that can be encompassed by 1 lumpectomy * No proven multicentric carcinoma (tumors in different quadrants of the breast or tumor separated by ≥ 4 cm) with other clinically or radiographically suspicious areas in the ipsilateral breast unless confirmed to be negative for malignancy by biopsy * No evidence of suspicious microcalcifications in the breast before the start of radiotherapy * If malignancy-associated microcalcifications were initially present, the post-excision mammography must be negative * No more than 9 positive axillary lymph nodes * No palpable or radiographically suspicious contralateral axillary, supraclavicular, infraclavicular, or internal mammary nodes unless th…