CLINICAL TRIAL / NCT00066703

DISEASE CHARACTERISTICS: - Histologically confirmed breast cancer - Completely resected disease - No clinically detectable residual loco-regional axillary disease - Prior surgery for primary breast cancer of 1 of the following types: - Total mastectomy with or without adjuvant radiotherapy - Breast-conserving procedure (e.g., lumpectomy, quadrantectomy, or partial mastectomy with margins negative* for invasive disease and ductal carcinoma in situ) with planned radiotherapy NOTE: *If all other margins are clear a positive posterior (deep) margin is permitted, provided the excision was performed down to the pectoral fascia and all tumor has been removed OR a positive anterior (superficial; abutting skin) margin is allowed provided all tumor was removed - Tumor confined to the breast and axillary nodes - Tumor detected in internal mammary chain nodes by sentinel node procedure and is not enlarged is allowed - Axillary lymph node dissection or a negative axillary sentinel node biopsy required - Patients with negative or microscopically positive axillary sentinel nodes are eligible - Positive sentinel nodes must have either axillary dissection or radiation of axillary nodes - No distant metastases - No locally advanced inoperable breast cancer, including any of the following: - Inflammatory breast cancer - Supraclavicular node involvement - Enlarged internal mammary nodes (unless pathologically negative) - Bilateral synchronous invasive breast cancer allowed if disease meets all other eligibility criteria - No prior ipsilateral or contralateral invasive breast cancer - Hormone receptor status: - Estrogen and/or progesterone receptor positive - At least 10% of the tumor cells positive by immunohistochemistry - If > 1 breast tumor, each tumor must be hormone receptor positive PATIENT CHARACTERISTICS: Age - Premenopausal Sex - Female Menopausal status - Premenopausal - Estradiol in the premenopausal range after prior surgery OR meets the following criteria: - Menstruating regularly for the past 6 months - Has not used any form of hormonal treatment (including hormonal contraception) within the past 6 months Performance status - Not specified Life expectancy - Not specified Hematopoietic - Not specified Hepatic - No systemic hepatic disease that would preclude prolonged follow-up Renal - No systemic renal disease that would preclude prolonged follow-up Cardiovascular - No systemic cardiovascular disease that would preclude prolonged follow-up - No prior thrombosis (e.g., deep vein thrombosis) and/or embolism unless patient is medically suitable Pulmonary - No systemic pulmonary disease that would preclude prolonged follow-up Other - Not pregnant or nursing - Fertile patients must use effective nonhormonal contraception - No history of noncompliance to medical regimens - No other nonmalignant systemic disease that would preclude prolonged follow-up - No other malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer, nonbreast carcinoma in situ, contralateral or ipsilateral carcinoma in situ of the breast, or other nonrecurrent invasive nonbreast malignancy, including any of the following: - Stage I papillary thyroid cancer - Stage IA carcinoma of the cervix - Stage IA or B endometrioid endometrial cancer - Borderline or stage I ovarian cancer - No psychiatric, addictive, or other disorder that would preclude study compliance PRIOR CONCURRENT THERAPY: Biologic therapy - Prior or concurrent neoadjuvant or adjuvant trastuzumab allowed Chemotherapy - No prior neoadjuvant or adjuvant chemotherapy Endocrine therapy - No prior tamoxifen, other selective estrogen-receptor modulators (SERMs) (e.g., raloxifene), or hormone replacement therapy for more than 1 year before breast cancer diagnosis - No prior neoadjuvant or adjuvant endocrine therapy since diagnosis of breast cancer - No concurrent oral or transdermal hormonal therapy - No other concurrent estrogen, progesterone, or androgens - No other concurrent aromatase inhibitors - No concurrent oral or other hormonal contraceptives (i.e., implants or depot injections) Radiotherapy - See Disease Characteristics - No prior ovarian radiotherapy Surgery - See Disease Characteristics - No prior bilateral oophorectomy Other - No concurrent bisphosphonates, except in the following cases: - Bone density is at least 1.5 standard deviations below the young adult normal mean - Participation in a randomized clinical study testing bisphosphonates in the adjuvant breast cancer setting - No other concurrent investigational agents