In patients with breast cancer who undergo neoadjuvant chemotherapy or neoadjuvant hormone therapy molecular breast imaging (MBI) is an accurate test for assessing response rate to neoadjuvant therapy.

The investigators propose to evaluate Molecular Breast Imaging in patients undergoing neoadjuvant therapy for breast cancer with the aims to document that 1) the change in uptake of 99mTc-sestamibi prior to initiation of neoadjuvant therapy, after 3-5 weeks of neoadjuvant therapy and at completion of neoadjuvant therapy prior to breast cancer surgery will be a reflection of the tumor response to neoadjuvant therapy.

2) Tumor response at 3-5 weeks of neoadjuvant therapy is predictive of tumor response at completions of neoadjuvant therapy.

3) Post-neoadjuvant tumor size as determined by MBI corresponds to size found at surgery.

Inclusion Criteria:

• Women with breast cancer scheduled to undergo neoadjuvant chemotherapy therapy or neoadjuvant hormone therapy and who have undergone any breast imaging procedure, and in whom a repeat imaging procedure is planned prior to definite surgery. MRI of the breast for accurate tumor size evaluation is recommended but not necessarily required for study inclusion.
• Are able to return for a 3-5 week follow-up MBI study and for the final MBI study at completion of the neoadjuvant therapy.
• Patient age > 18

Exclusion Criteria:

• Unable to understand or sign a consent form
• Pregnant or lactating
• Physically unable to sit upright and still for 40 minutes

20 patients who present to the Internal Medicine Breast Cancer Clinic with breast cancer and who after discussion with the consulting surgeon and oncologist have agreed to undergo neoadjuvant chemotherapy or neoadjuvant hormone therapy.