This pilot observational study is being done to identify possible biomarkers of response to
endocrine therapy in patients with ER/PR+ metastatic lobular breast cancer (LBC) starting new
endocrine therapy. 18F-fluorofuranylnorprogesterone Positron Emission Tomography/Computed
Tomography (FFNP-PET/CT) and liquid biopsies will be performed at baseline and after 4 weeks
of treatment. Baseline levels and dynamic on-treatment changes in estrogen signaling as
measured by FFNP-PET/CT and circulating tumor cell (CTC) liquid biopsy will be correlated
with clinical response to endocrine therapy and progression-free survival in the above cohort
of patients.