Inclusion Criteria:

• Ages 40 - 55 years
• Considered premenopausal or peri-menopausal
• Screening mammogram performed at Mayo Clinic Rochester within one year prior to the current MBI study which demonstrates
• Negative/benign assessment (BIRADS category 1-2) or Additional imaging recommended (BIRADS category 0), where the subsequent diagnostic workup results in a final negative/benign assessment (BIRADS category 1-2).
• No proliferative benign lesions (e.g. fibroadenomas) identified
• Heterogeneously dense or extremely dense parenchyma (BIRADs density category 3 or 4)
• MBI performed less than 3 days prior to biopsy demonstrating either marked FD or photopenic FD in an area of mammographically dense tissue amenable to biopsy.
• No use of exogenous hormonal drugs (e.g. hormonal contraceptives or sex steroid hormones) within six months prior to study biopsy
• Negative pregnancy test or surgically sterilized (for patients in whom a study MBI will be performed)

Exclusion criteria:

• Using any estrogen receptor modulating drugs (e.g., tamoxifen, raloxifene) or any aromatase inhibitors within six months prior to study biopsy.
• Prior diagnosis of bilateral breast cancer.
• Diagnosis of breast cancer in either breast within 3 years prior to study MBI.
• Diagnosis of any cancer, except breast cancer (see #3) or non-melanomatous skin cancer, for which patient has not been disease free for at least 5 years.
• Current breast symptoms
• Breast implants
• Known allergy to local anesthetic.
• History of bleeding complications from prior interventions
• Current use of anticoagulants (e.g., Coumadin or other blood thinners)
• Major medical condition