The purpose is to evaluate whether non invasive in vivo imaging of the estrogen receptor (ER)
presence in metastatic breast cancer patient by means of 18F-fluoro-estradiol (FES) positron
emission tomography (PET) can be used to predict treatment response to palbociclib plus
letrozole. As ER expression predicts response to palbociclib in metastatic breast cancer
patients the investigators hypothesize that lesions with low uptake on FES-PET will not
respond to the combination of letrozole plus palbociclib.