Dual Tracers of 16-[18F]fluoro-17-Estradiol and [18F]fluorodeoxyglucose for Prediction of Progression-Free Survival After Fulvestrant Therapy in Patients With HR+/HER2-Metastatic Breast Cancer

Objective The purpose of this study was to employ dual tracers 16 alpha-[18F]fluoro-17 beta-estradiol (F-18-FES) and [18F]fluorodeoxyglucose (F-18-FDG) as imaging biomarkers in predicting progression-free survival (PFS) in ER-positive metastatic breast cancer (MBC) patients receiving fulvestrant therapy. Methods We retrospectively analyzed 35 HR+HER2- MBC patients who underwent F-18-FES and F-18-FDG PET/CT scans prior to fulvestrant therapy in our center. The SUVmax across all metastatic lesions on the PET/CT were assessed. The heterogeneity of ER expression was assigned by the presence of any F-18-FES negative lesions for patients with entirely F-18-FES positive lesions categorized into two groups by the median ratio of FES/FDG SUVmax, low FES/FDG, and high FES/FDG. PFS were estimated by the Kaplan-Meier method and compared by the log-rank test. Univariate and multivariate analyses were performed using the Cox proportional hazard model. Results In total, 12 patients had both F-18-FES negative and positive lesions, indicating the heterogeneity of ER expression in metastatic lesions. These patients had a low median PFS of 5.5 months (95% CI 2.3-8.7). Of patients with entirely F-18-FES positive lesions, 11 had a low FES/FDG, and 12 had a high FES/FDG. These groups had a median PFS of 29.4 months (95% CI 2.3-56.5) and 14.7 months (95% CI 10.9-18.5), respectively. The patients were stratified in three categories based on incorporating both F-18-FES and F-18-FDG imaging results that were significantly correlated with PFS by univariate analysis (P < 0.001) and multivariate analysis (P = 0.006). Conclusion F-18-FES and F-18-FDG PET could serve as prognostic imaging biomarkers for ER-positive MBC patients treated with fulvestrant therapy.