Inclusion Criteria:

- Females with stage 0-3 breast cancer who have had or are planned to have a breast MRI
within 1 year prior to starting tamoxifen

- Premenopausal status is defined as intact ovaries and still menstruating

- Clinical and MRI follow-up planned at MSKCC or MCKCC regional facility

- Willing and able to undergo all study procedures

Exclusion Criteria:

- Contraindication to breast MRI (such as non-compatible cardiac pacemakers or
intracranial vascular clips, allergy to Gadolinium)

- Bilateral breast cancer or treatment such as:

- History of or planned bilateral breast irradiation

- History of or planned bilateral mastectomy

- Bilateral breast cancer

- History of unilateral mastectomy or radiation treatment with contralateral breast
cancer

- Taking chemo- or hormonal therapy at the time of the baseline breast MRI

- Estrogen and progesterone receptor negative breast cancer

- GFR less than GFR < 30 mL/min/1.73m2

- Postmenopausal women

- Pregnant and/or nursing women

- Less than 21 years of age