ESR1 and breast cancer: A prospective study involving 90 ER+ BC patients treated with endocrine therapy found that [18F]-FES-PET(/CT) may be a useful prognostic biomarker for [18F]-FDG avid tumors, demonstrating a higher median progression-free survival (PFS) in the high [18F]-FES uptake group compared to low [18F]-FES uptake group (7.9 versus 3.3 months, respectively) [48].