Studying 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.