RATIONALE: Chemoprevention is the use of certain drugs to keep cancer from forming. The use of soy isoflavones supplements may prevent or treat early stage breast cancer. PURPOSE: This clinical trial studies soy isoflavones supplementation in treating women at high risk for or with breast cancer.

PRIMARY OBJECTIVES: I. To assess whether magnetic resonance imaging (MRI) volume (equivalent to 3-dimensional mammographic density) is reduced in high-risk women or those with invasive breast cancer or DCIS who are supplemented daily with soy (5p mg total isoflavones as aglycone) compared to placebo (microcrystalline cellulose) tablets for 1 year.II. To assess whether cell proliferation and apoptosis, as measured by Ki67 and caspase 3 staining, respectively, of breast epithelial cells is altered with soy treatment.SECONDARY OBJECTIVES: I. To assess whether other intermediate molecular markers including estrogen receptor alpha (ER alpha) and ER beta differ between women supplemented with soy vs placebo. OUTLINE: Patients are randomized to 1 of 2 treatment arms.ARM I: Patients receive oral soy isoflavones supplement once daily for12 months in the absence of disease progression.ARM II: Patients receive oral placebo once daily for 12 months in the absence of disease progression.

Inclusion Criteria:

• Women at high risk for breast cancer, defined as any of the following groups:
• Five year Gail risk > 1.7%
• Known BRCA1/BRCA2 mutation carrier
• Family history consistent with hereditary breast cancer
• Prior biopsy exhibiting atypical hyperplasia or lobular carcinoma in situ (LCIS)
• History of invasive breast cancer or ductal carcinoma in situ (DCIS) and have completed standard therapy including tamoxifen/aromatase inhibitor or will not be treated with tamoxifen/aromatase inhibitor
• Signed Informed Consent

Exclusion Criteria:

• Metastatic breast cancer
• Undergoing treatment (chemotherapy, radiation, or SERMs)
• Pregnancy or breast-feeding, or planning to become pregnant within one year prior to study entry
• Regular soy consumers (i.e., < once per week of soy food, soy supplements or other products)
• Known food allergies such as to soy or nuts
• Not willing to avoid soy foods/supplements during study period
• Current users of exogenous hormones or oral contraceptive or planning to use exogenous hormones during the duration of the study
• Cannot stop taking aspirin or NSAIDs within a week of breast biopsy
• Active participant in other ongoing trials