Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed breast cancer meeting 1 of the following criteria:

- Ductal carcinoma in situ (DCIS)

- Invasive carcinoma

- Stage I-III disease

- Must have undergone breast cancer surgery within the past 6 months, including any of
the following:

- Mastectomy or lumpectomy with or without radiation

- Sentinel node and/or axillary node dissection

- Re-excision of lumpectomy margins

- Intact contralateral breast

- No prior radiation therapy or mastectomy

- Prior biopsies allowed

- Hormone receptor status:

- Estrogen and/or progesterone receptor-positive tumor by immunohistochemical
staining

PATIENT CHARACTERISTICS:

Sex

- Female

Menopausal status

- Postmenopausal, defined as 1 of the following:

- At least 60 years of age

- Less than 60 years of age and amenorrheic ≥ 12 months prior to study entry

- Less than 60 years of age, amenorrheic for < 12 months prior to day 1, and
luteinizing hormone (LH) and follicle stimulating hormone (FSH) values within
postmenopausal range

- Less than 60 years of age, without a uterus, and LH and FSH values within
postmenopausal range

- Underwent prior bilateral oophorectomy

- Underwent prior radiation castration AND amenorrheic for ≥ 6 months

Performance status

- ECOG 0-2

Life expectancy

- Not specified

Hematopoietic

- Not specified

Hepatic

- Not specified

Renal

- Not specified

PRIOR CONCURRENT THERAPY:

Chemotherapy

- No prior or concurrent adjuvant chemotherapy for breast cancer

Endocrine therapy

- No prior aromatase inhibitor

- At least 6 weeks since prior and no concurrent tamoxifen

- At least 6 weeks since prior and no concurrent hormone replacement therapy

- Concurrent low-dose, local vaginal estrogen in the form of either a conjugated
estrogen ring or other similar product (e.g., Estring® or Vagifem®) allowed

- At least 6 weeks since prior and no other concurrent selective estrogen receptor
modulators (e.g., raloxifene)

Radiotherapy

- See Disease Characteristics

Surgery

- See Disease Characteristics

- No contralateral breast implant

Other

- Concurrent bisphosphonates allowed at the discretion of the treating oncologist

- No concurrent consumption of soy supplements

- Concurrent routine dietary consumption of soy-containing foods allowed

- No other concurrent investigational or commercial drugs or therapies for the
treatment of DCIS or invasive breast cancer