The Use of FES-PET Imaging as a Tool to Detect a Possible Reversion of Estrogen Receptor (ER)-α Status in Patients With Metastatic Breast Cancer HER2 + and ERα Neg Treated With Trastuzumab + Pertuzumab + Taxane.

Inclusion Criteria: 1. Age ≥ 18 years. 2. Patient with metastatic breast cancer HER2 + (IHC+++ and/or HER2 amplification in ISH according to ASCO recommendations), ERα neg (0% in IHC) and Progesterone Receptor neg (0% in IHC). 3. Patient eligible according to the investigator for a treatment with trastuzumab + pertuzumab + taxane in the metastatic first line. 4. Available biopsy of a tumor lesion (archived material) or biopsiable tumor lesion for study (primitive tumor or metastasis other than bone). 5. OMS ≤ 2. 6. For non-menopausal patients, use of an effective contraceptive method at entry into the study and for the duration of the study. 7. Patient affiliated to a Social Health Insurance in France. 8. Patient must provide written informed consent prior to any study specific procedures. Exclusion Criteria: 1. Any previous treatment for metastatic disease. 2. Prior adjuvant treatment with anti-HER2 antibodies taken within 6 months. 3. Patient with isolated hepatic metastasis. 4. Patient with hemostasis disorders. 5. Unbalanced Diabetes. 6. Patient with usual formal contraindication to PET/TDM Imaging. 7. Patient who has already started trastuzumab + pertuzumab + taxane treatment. 8. Pregnant or breastfeeding women. 9. Any psychological, familial, geographical or sociological condition which does not allow to respect the medical follow-up and/or compliance to study procedure. 10. Patient protected by law.