Inclusion Criteria:

• Postmenopausal patients, defined as one or more of the following criteria:

Documented history of bilateral oophorectomy, Aged 60 years or more,

Aged 45-59 years and satisfying one or more of the following criteria:

Amenorrhea for at least 12 months and intact uterus Amenorrhea for less than 12 months and follicle stimulating hormone (FSH) concentrations within postmenopausal range including: patients who have had a hysterectomy and patients who have received hormone replacement therapy in the past.

• Clinically palpable or non-palpable T1c or greater, ER positive invasive breast cancer diagnosed by outside core biopsy.
• Outside diagnosis of invasive breast cancer confirmed at MSKCC
• Palpable or non-palpable breast mass > 1cm highly suspicious for invasive breast cancer based on mammographic, ultrasound and/or physical examination findings and amenable to diagnostic core biopsy.
• Core biopsy diagnosis of ER positive invasive breast cancer performed at MSKCC
• Breast conserving surgery or mastectomy scheduled at MSKCC.
• Informed consent obtained.

Exclusion Criteria:

• Co-morbid conditions which would preclude use of aromatase inhibitors such as: previous hypersensitivity, severe hepatic insufficiency (SGOT or SGPT three times the upper limit of normal), severe renal insufficiency (creatinine clearance <10ml/min).

Prior radiation therapy to chest wall / breast.

• Neoadjuvant chemotherapy
• Hormone replacement therapy, tamoxifen or raloxifene treatment within past 3 months
• ER negative breast cancer
• History of active malignancy within the previous 5 years (except for nonmelanoma skin cancer and breast cancer)