Exemestane or Anastrozole in Treating Postmenopausal Women Who Have Undergone Surgery for Primary Breast Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed invasive breast cancer * pT1-3; pNX, pN0-2 or pN3\*; M0 * Neoadjuvant patients are eligible no earlier than 3 weeks or later than 3 months after excisional surgery, provided both the clinical-diagnostic staging of cancer and postsurgical resection-pathologic staging of cancer meet the requirements for primary tumor, regional lymph nodes, and distant metastasis classification NOTE: \*Only when the sole basis for this classification is the presence of 10 or more involved axillary lymph nodes * Completely resected disease * Primary surgery performed at least 3 weeks but no more than 3 months before study entry (if no chemotherapy was given) * Primary surgery is defined as the last surgery at which histologic evidence of invasive or in situ disease was present in the pathology specimen * Patients with positive sentinel lymph node biopsy are eligible provided they have had a subsequent axillary lymph node dissection * No metachronous breast cancer * Bilateral mammogram within the past 12 months unless initial surgery was a total mastectomy, in which case only a mammogram of the remaining breast is required * No metastases confirmed by 1 of the following methods: * Bone scan\* (required only if alkaline phosphatase is at least 2 times normal and/or there are symptoms of metastatic disease) * Abdominal ultrasound or CT scan (required only if AST/ALT or alkaline phosphatase is at least 2 times normal, unless the elevation is in the bone fraction) * Chest x-ray NOTE: \*Confirmatory x-ray, CT scan, or MRI required if the bone scan results are questionable * No locally recurrent disease * No prior or concurrent carcinoma in situ of the contralateral breast treated with partial mastectomy and/or hormonal therapy * Patients with prior or concurrent carcinoma in situ of the ipsilateral breast are eligible provided the tumor was completely excised AND they have not received prior hormonal therapy * Hormone receptor status: * Estrogen receptor- and/or progesterone receptor-positive by immunohistochemistry or tumor receptor content ≥ 10 fmol/mg protein PATIENT CHARACTERISTICS: Age * Postmenopausal Sex * Female Menopausal status * Postmenopausal prior to chemotherapy, defined as 1 of the following: * Over 60 years of age * Age 45-59 with spontaneous cessation of menses for more than 1 year prior to study entry * Age 45-59 with menses ceasing (secondary to hysterectomy or spontaneously) within the past year AND a follicle-stimulating hormone (FSH) level prior to study entry in the postmenopausal range\* * Age 45-59, previously on hormone replacement therapy (HRT) and have discontinued HRT upon diagnosis of this malignancy AND has an FSH level prior to study entry in the postmenopausal range\* * Has undergone bilateral oophorectomy NOTE: \*By institutional standards OR \> 34.4 IU/L if institutional range is not available) Performance status * ECOG 0-2 Life expectancy * At least 5 years Hematopoietic * WBC at least 3,000/mm\^3 OR * Granulocyte count at least 1,500/mm\^3 AND * Platelet count at least 100,000/mm\^3 Hepatic * See Disease Characteristics * AST and/or ALT less than 2 times upper limit of normal (ULN)\* * Alkaline phosphatase less than 2 times ULN\* NOTE: \*Unless imaging examinations have ruled out metastatic disease Renal * Not specified Other * Able to swallow study medication and have adequate unassisted oral intake in order to maintain reasonable nutrition status * No other non-breast malignancy within the past 5 years except adequately treated nonmelanoma skin cancer, curatively treated carcinoma in situ of the cervix, or other curatively treated solid tumors with no evidence of disease for at least 5 years * No other concurrent medical or psychiatric condition that would preclude study participation and/or interfere with results PRIOR CONCURRENT THERAPY: Biologic therapy * Prior and concurrent trastuzumab (Herceptin®) allowed Chemotherapy * See Disease Characteristics * At least 3 weeks but no more than 3 months since prior chemotherapy * Prior adjuvant chemotherapy allowed Endocrine therapy * See Disease Characteristics * No prior aromatase inhibitor * No prior tamoxifen or other selective estrogen receptor modulators (SERMs) except raloxifene * At least 3 weeks since prior raloxifene * At least 3 weeks since prior and no concurrent over-the-counter products or supplements considered to have an estrogenic effect, including any of the following: * Ginseng * Ginkgo biloba * Black cohosh * Dong quai * Fortified soy supplements (e.g., phytoestrogen preparations) * At least 3 weeks since other prior hormonal therapy or steroids considered to have an estrogenic effect * No concurrent estrogens, progesterones, androgens, or SERMs * Concurrent intermittent vaginal estrogens (e.g., vagifem, estrogen vaginal cream, testosterone, estradiol vaginal gel, or Estring) allowed if other local measures for intractable vaginal atrophy are insufficient * No other concurrent therapy that would have an estrogenic effect, including endocrine therapy, hormonal therapy, or steroid therapy Radiotherapy * See Disease Characteristics * Prior adjuvant radiotherapy allowed * Concurrent radiotherapy allowed Surgery * See Disease Characteristics