Dr. Hunter on the Role of 18F-FES-PET Imaging in Triple-Positive Breast Cancer

Natasha B Hunter, MD, assistant professor, Division of Medical Oncology, University of Washington School of Medicine, affiliate professor, Clinical Research Division, Fred Hutchinson Cancer Center, discusses the utility of 18F-Fluoroestradiol (FES) PET imaging for patients with estrogen receptor (ER)–positive and HER2-positive breast cancer.

The recent FDA approval of 18F-FES-PET imaging for the characterization of ER-positive breast cancer necessitates continued research on its specific applications within hormone receptor (HR)–positive breast cancer subtypes, Hunter begins. The tracer has been proven to indicate ER expression and is correlated with a patient’s response to endocrine therapy.

A retrospective study was conducted to assess the utility of FES-PET and 18F-Fluorodeoxyglucose (FDG) PET in determining ER function and expression in triple-positive patients, Hunter says. Preliminary data were previously presented at the 2022 ASCO Annual Meeting and showed that paired FES-PET and FDG-PET scans were comparable in uptake, while HER2-positive patients demonstrated increased FES positivity.

Subsequent subgroup analysis of patients from this trial was performed, Hunter continues. This study evaluated matched lesion uptake and activity with both FES-PET and FDG-PET in patients with ER-positive, HER2-positive vs HER2-negative metastatic breast cancer. Data were presented at the 2022 San Antonio Breast Cancer Symposium and showed that FDG and FES activity was comparable in all groups regardless of HER2 status. Moreover, matched lesion uptake with FES occurred in both HER2-positive and HER2-negative groups and was fairly stable over time, suggesting that HER2 status does not impact ER density, she states. Although HER2 is often considered the sole driver in HER2-positive disease, these results indicate that the ER pathway is highly active in the triple-positive population and that there’s crosstalk between the ER pathway and HER2 pathway, Hunter notes.

Ultimately, these data suggest that FES-PET may be a useful tool for navigating treatment selection, particularly the early administration of endocrine therapy, in the ER-positive, HER2-positive subset, Hunter explains. It may also allow for improved identification of heterogeneous tumors in discordant scans, thereby revealing key biopsy sites, she concludes.