Letrozole in Treating Postmenopausal Women Who Have Received Hormone Therapy for Hormone Receptor-Positive Breast Cancer

OBJECTIVES: Primary * Determine whether or not prolonged adjuvant hormonal therapy comprising letrozole vs placebo will improve disease-free survival of postmenopausal women with estrogen receptor-positive and/or progesterone receptor-positive breast cancer who have completed 5 years of hormonal therapy with 5 years of an aromatase inhibitor (AI) or 5 years of a combination of up to 3 years of tamoxifen citrate followed by an AI. * Compare the disease-free survival of patients treated with these regimens. Secondary * Compare overall survival of patients treated with these regimens. * Compare breast cancer-free interval of patients treated with these regimens. * Compare distant recurrence in patients treated with these regimens. * Compare the incidence of osteoporotic-related fractures (e.g., Colles', hip, and spine) in these patients treated with these regimens. * Compare the incidence of arterial thrombotic events in patients treated with these regimens. OUTLINE: This is a double-blind, multicenter, placebo-controlled, randomized study. Patients are stratified according to pathologic nodal status (negative vs positive), adjuvant tamoxifen citrate therapy (yes vs no), and lowest bone mineral density T score for lumbosacral spine, total hip, or femoral neck (\> -2.0 vs ≤ -2.0 standard deviation). Patients are randomized to 1 of 2 treatment arms. * Group I: Patients receive oral placebo once daily. * Group II: Patients receive oral letrozole once daily. In both arms, treatment continues for up to 5 years in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed annually. PROJECTED ACCRUAL: A total of 3,840 patients will be accrued for this study.