Compare the response rate after 6 months of hormone treatment (or a major change in hormone treatment) in metastatic breast cancer, according to the uptake of FES in metastatic lesions taking-up FDG on PET/CT at baseline. Hypothesis: best response rate will be observed in patients with all metastases taking up FES prior to treatment.

Main objective: Compare the response rate after 6 months of hormone treatment (or a major change in hormone treatment) in metastatic breast cancer, according to the uptake of FES in metastatic lesions taking-up FDG on PET/CT at baseline.

Secondary objectives:

• evaluate diagnostic performance of FES PET/CT
• determine whether FES PET/CT is able to detect metastases that are not visible on FDG PET/CT. This point may constitute a direct benefit for the patient
• precise the nature of discordant FES/FDG foci
• validate and improve the interpretation criteria for FES PET/CT
• confirm the perfect tolerance

Inclusion Criteria:

• Post menopausal
• age > 17
• WHO 0-2
• Metastatic adenocarcinoma of the breast
• Treated by antihormone treatment during around 5 years withdrawn for at least 3 months OR metastatic cancer at diagnosis having received no more than one line of hormone treatment
• Life expectancy > 6 months
• Hormone-dependent cancer initially demonstrated by hormone receptors in the tumour
• Presence of oestrogen receptors proven with immunohistochemistry (> 10%) and HER2 determined by immunohistochemistry or FISH (on primary tumour or a metastasis)
• Metastatic recurrence on FDG PET dating less than 1 month, confirmed by another modality (contrast-enhanced CT, MRI, ultrasonography, bone scintigraphy or PET/CT, other)
• FDG PET/CT available on PACS or CD DICOM III format 11
• Informed consent obtained

Exclusion Criteria:

• Other evolutive malignant disease or acute or chronic infectious disease
• Chemotherapy during the last 3 months or change in treatment since FDG PET/CT.
• Isolated liver metastasis (high FES uptake by normal liver)