RATIONALE: Learning about the effect of metformin hydrochloride in breast density of women with early-stage breast cancer may help plan treatment.

PURPOSE: This trial studies changes in breast density in patients with early-stage breast cancer treated with metformin hydrochloride or placebo on CAN-NCIC-MA.32.

OBJECTIVES:

Primary

• To evaluate the change in percent mammographic density in contralateral (unaffected breast) from prior to the initiation of metformin hydrochloride (metformin) or placebo treatment through one year of therapy in patients with hormone receptor-negative breast cancer (i.e., not on endocrine therapy).

Secondary

• To evaluate the change in percent breast density in contralateral (unaffected breast) from prior to the initiation of metformin or placebo treatment through two years of therapy in patients with hormone receptor-negative breast cancer (i.e., not on endocrine therapy).
• To evaluate whether baseline mammographic density correlates with baseline of fasting plasma insulin, glucose levels, and Homeostasis Model Assessment (HOMA) (collected on the treatment protocol CAN-NCIC-MA.32) (MA.32).
• To evaluate whether changes in dense area in response to metformin therapy from pre-treatment through two years of therapy correlate with changes in fasting plasma insulin, glucose levels, and HOMA (collected on the treatment protocol MA.32) over the same time period.
• To explore whether change in mammographic density for women on metformin is associated with risk of second primary breast cancer.

OUTLINE: Patients' mammograms taken at baseline and at approximately 1 and 2 years of metformin or placebo treatment are retrieved and analyzed for breast-density change. Pre-menopausal patients' menstrual cycle information is also collected at baseline and every 6 months for 2 years.

DISEASE CHARACTERISTICS:

• Patients must either be concurrently enrolling or previously enrolled to Canada (CAN) National Cancer Institute of Canada (NCIC) study MA.32 (CAN-NCIC-MA.32) (MA.32); eligible patients may be either pre- or post-menopausal
• Patients must have hormone receptor-negative breast cancer
• Patients must have breast density ≥ 25% (correlating with the Breast Imaging-Reporting and Data [BIRAD]-2 category of "scattered fibroglandular densities" or greater)
• Baseline digital mammograms taken within 12 months prior to registration to MA.32, with at least a craniocaudal (CC) view used for enrollment to MA.32 must be available for submission; if the patient has previously enrolled to MA.32 and one year has elapsed from baseline mammograms, one-year mammograms must also be available for submission
• Contralateral unaffected breast in place (with no prior cancer or radiation, no implants, and no plan for breast surgery on contralateral breast over the course of the study); women with a prior biopsy on the unaffected breast are eligible

PATIENT CHARACTERISTICS:

• Not specified

PRIOR CONCURRENT THERAPY:

• See Disease Characteristics
• Women receiving endocrine therapy (e.g., tamoxifen, aromatase inhibitors) are not eligible