RATIONALE: Learning about the effect of exemestane and anastrozole on bone mineral density in postmenopausal women with primary breast cancer may help plan treatment, decrease the risk of broken bones, and help patients live more comfortably.

PURPOSE: This phase III trial is studying bone mineral density in postmenopausal women with primary breast cancer who are receiving treatment on clinical trial CAN-NCIC-MA27.

OBJECTIVES:

• Determine whether there is a clinically relevant difference in bone mineral density (BMD) at 2 years in postmenopausal women with primary breast cancer (with or without osteopenia or osteoporosis) treated with exemestane vs anastrozole on protocol CAN-NCIC-MA27.

OUTLINE: This is a multicenter, companion study. Patients are stratified according to baseline bone mineral density (BMD) measurement (T-score* ≥ -2.0 standard deviation [SD] [no osteopenia or osteoporosis] vs T-score* < -2.0 SD).

NOTE: *The lowest of the two T-scores: L1-L4 or total hip

Blood samples for the identification of bone biomarkers (formation marker: serum amino-terminal procollagen 1 extension peptide [P1NP] and resorption marker: serum N-telopeptide) are obtained at baseline and at 6 and 12 months. BMD is determined by dual-energy x-ray absorptiometry (DEXA) at baseline and then annually for 5 years (or for as long as patient is receiving treatment on protocol CAN-NCIC-MA27).

Patients receive oral calcium and oral cholecalciferol (vitamin D) daily. Patients with osteopenia or osteoporosis (stratum II) also receive oral bisphosphonate therapy

PROJECTED ACCRUAL: A total of 408 patients will be accrued for this study.

DISEASE CHARACTERISTICS:

• Enrolled in and meets eligibility requirements for protocol CAN-NCIC-MA27

• Acceptable quality dual-energy x-ray absorptiometry (DEXA) of the L1-L4 postero-anterior spine and hip within 12 weeks prior to randomization on protocol CAN-NCIC-MA27

• Hormone receptor status:

  • Estrogen receptor- and/or progesterone receptor-positive tumor

PATIENT CHARACTERISTICS:

• Female
• Postmenopausal
• No malabsorption syndrome
• No known cholecalciferol (vitamin D) deficiency, active hyper- or hypoparathyroidism, or Paget's disease
• No uncontrolled thyroid disease, Cushing's disease, or other pituitary disease
• No other bone disease (including osteomalacia or osteogenesis imperfecta)

PRIOR CONCURRENT THERAPY:

• More than 6 months since prior drugs (investigational or not), including bisphosphonates, for the prevention of osteoporosis (stratum I)
• More than 12 months since prior and no concurrent anticonvulsants
• More than 6 months since prior and no concurrent corticosteroids at doses > 5 mg/day of prednisone (or equivalent) for > 2 weeks
• More than 12 months since prior and no concurrent anabolic steroids
• No prior bisphosphonates (stratum II)
• No concurrent sodium fluoride at daily doses ≥ 5 mg/day
• No long-term (i.e., > 6 months) use of coumarins
• No concurrent drugs (investigational or not), including bisphosphonates, for the prevention of osteoporosis (for patients with no osteopenia or osteoporosis [stratum I])