ESR1 and invasive lobular breast carcinoma: These applications include assessing GRPR expression to identify patients who are more likely to respond to GRPR-targeted radionuclide therapies; non-invasive assessment of ER expression in metastatic lesions; therapy response monitoring; imaging of invasive lobular carcinoma metastases, which often have low uptake of [18F]-FDG but high expression of ER.