Anastrozole or Tamoxifen in Treating Postmenopausal Women With Ductal Carcinoma in Situ Who Are Undergoing Lumpectomy and Radiation Therapy

DISEASE CHARACTERISTICS: * Histologically confirmed ductal carcinoma in situ (DCIS) of the breast * Mixed DCIS and lobular carcinoma in situ (LCIS) allowed * Must have undergone lumpectomy * Margins must be histologically free of disease * Re-excision to obtain tumor-free margins allowed * No more than 84 days since prior lumpectomy or re-excision * More than 1 area of DCIS allowed provided all disease is removed with tumor-free margins * Masses or clusters of calcification that are clinically or mammographically suspicious must be biopsied * No prior invasive breast cancer or DCIS * Patients with a history of LCIS are eligible * No prior or concurrent invasive (including microinvasive) breast cancer * DCIS "suspicious" for microinvasion allowed * No bilateral malignancy * No mass or mammographic abnormality suspicious for malignancy in the opposite breast unless not malignant as proven by biopsy * No Paget's disease of the nipple * No positive ipsilateral axillary or intramammary nodes * No palpable nodes in the ipsilateral or contralateral axilla or palpable supraclavicular or infraclavicular nodes unless not involved with tumor as proven by biopsy * Hormone receptor status: * Estrogen- or progesterone-receptor positive as determined by immunohistochemistry * Borderline results are considered positive PATIENT CHARACTERISTICS: Age * See Menopausal status Sex: * Female Menopausal status: * Postmenopausal as defined by at least 1 of the following: * Prior documented bilateral oophorectomy * At least 12 months without spontaneous bleeding * Age 55 or over with prior hysterectomy without oophorectomy * Age 54 or under with prior hysterectomy without oophorectomy with a documented follicle-stimulating hormone level in the postmenopausal range Performance status * Zubrod 0-2 Life expectancy * At least 10 years (excluding diagnosis of breast cancer) Hematopoietic * WBC normal Hepatic * AST normal * Bilirubin normal * Alkaline phosphatase normal * No hepatic disease that would preclude administration of study drugs Renal * Creatinine normal * No renal disease that would preclude administration of study drugs Cardiovascular * No prior documented cerebral vascular accident or transient ischemic attack * No prior deep vein thrombosis * No cardiovascular disease that would preclude administration of study drugs * No uncontrolled hypertension (i.e., systolic blood pressure at least 180 mm Hg or diastolic blood pressure at least 110 mm Hg based on the average of 2 or more readings at each of 2 or more visits after initial screening) * No uncontrolled atrial fibrillation Pulmonary * No pulmonary embolus Other * Not pregnant or nursing * Patients with a history of non-breast malignancies are eligible provided they have been disease-free for ≥ 5 years and are deemed by their physician to be at low risk for recurrence * No other malignancy within the past 5 years except treated basal cell or squamous cell skin cancer, carcinoma in situ of the cervix, carcinoma in situ of the colon, or melanoma in situ * No psychiatric or addictive disorders that would preclude informed consent * No uncontrolled diabetes, defined as hemoglobin A1C greater than 9% (fasting glucose 200 mg/dL) * No nonmalignant systemic disease that would preclude administration of study drugs PRIOR CONCURRENT THERAPY: Endocrine therapy * No prior or concurrent aromatase inhibitors (e.g., exemestane or letrozole) or tamoxifen * No concurrent raloxifene or other selective estrogen receptor modulators * No concurrent sex hormone therapy (e.g., estrogen or progesterone replacement therapy, oral contraceptives, androgens, luteinizing hormone releasing hormone analogs, prolactin inhibitors, or antiandrogens) * Low-dose estrogen vaginal creams or Estring allowed Radiotherapy * Radiotherapy for this cancer initiated before study is allowed Surgery * See Disease Characteristics * No prior or concurrent mastectomy for DCIS * Prior sentinel node biopsy or axillary node dissection allowed provided nodes are pathologically negative Other * No concurrent warfarin * No other systemic therapy for this cancer initiated before study * No other concurrent anticancer therapy unless permitted by the protocol investigator * No concurrent participation in another clinical trial of therapy for DCIS * Concurrent participation in protocol NSABP-B-39 allowed