Aromatase Inhibitors (AI) are effective for secondary prevention of breast cancer and may soon replace tamoxifen as first-line therapy in the treatment of hormone-sensitive breast cancer. However, because these medications produce a marked reduction in serum estrogen levels, this is likely to result in an increased rate of bone loss and risk of developing osteoporosis and fractures in postmenopausal women treated with these agents. Indeed, substantial bone loss has been reported in several large clinical trials of AIs. Osteoporosis drugs are available that could prevent this loss, but they have frequent side effects and are expensive. Thus, treating all women receiving AIs might not be the most appropriate and cost-effective approach. A better approach might be to select women at highest risk of bone loss and only treat them with antiresorptive agents.

The proposed pilot study will evaluate women who receive anastrozole or letrozole therapy, are receiving adequate amounts of calcium and vitamin D and have baseline normal or moderately low bone mass in order to determine if early changes in bone turnover markers correlate with bone loss at one year. If data from this pilot protocol support our hypothesis, then we would propose a larger trial to confirm it. The ultimate aim is to predict which women are at higher risk of bone loss and therefore treat them earlier with bone-sparing agents, while those with lower risk could be monitored on conservative therapy.

Inclusion Criteria:

• Postmenopausal women with diagnosis of breast cancer - have not started Arimidex or Femara yet, but will be starting .

Exclusion Criteria:

• History of metastasis
• History of chronic kidney
• Liver GI disease
• Disorders affecting calcium metabolism

Women receiving treatment for breast cancer, who will be starting Arimidex or Femara