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)