Abstract P2-03-17: Fluoroestradiol (FES) and Fluorodeoxyglucose (FDG) PET imaging in patients with ER+, HER2+ or HER2- metastatic breast cancer

Abstract Background: 18F-Fluorodeoxyglucose (FDG) has long been used for measuring tumor glycolytic activity in clinical PET imaging. The FDA recently approved 18F-Fluoroestradiol (FES) (Cerianna) as a PET imaging tracer for characterizing disease in patients with estrogen-receptor positive (ER+) breast cancer. As FES PET enters clinical practice it is important to establish its utility in the full population of hormone-receptor positive patients, including for patients with human epidermal growth factor 2 (HER2)-overexpressing tumors. Patients with HER2-positive metastatic breast cancer have historically been treated with combination cytotoxic and HER2-directed therapy, with the understanding that HER2 is the primary driver in this disease state. This retrospective study examined uptake in matched lesions for both FES and FDG PET and compared activity in patients with HER2 positive versus HER2 negative metastatic breast cancer. Methods: Patients were selected from the UW research database who had a history of biopsy-proven primary ER+ breast cancer as well as FES and FDG PET scans within 30 days. We examined FDG and FES scans and recorded SUVmax in up to 16 matched lesions between the two scans. Patients were also divided by HER2 status (+/-). In addition, a subset of patients who underwent at least 2 paired FDG and FES scans were reviewed. Results: 270 matched FDG and FES scans were analyzed in 216 patients with history of ER+, and HER2+ or HER2- breast cancer who were not part of an ongoing clinical trial. 158 (73%) had ductal disease, 38 (18%) had lobular disease. 183 (85%) had HER2- breast cancer. Of the 33 patients who had HER2+ breast cancer, 28 (85%) had ductal carcinoma. 40 patients underwent serial scans, allowing tracking over multiple timepoints. A total of 1323 metastatic sites were recorded (average = 5/scan (range 1,16)), with the majority (71%) representing bony lesions. No difference in quantitative FES or FDG avidity was observed between soft tissue and osseous sites. FES and FDG SUVmax were similar among patients with either HER2- or HER2+ breast cancer (Table 1). Among 40 patients with multiple paired FDG and FES scans, 26 (65%) had 2 scans while the remaining 14 had 3, with FES avidity remaining stable over time. There was no correlation between FES or FDG scans and HER2 status. Conclusion: In a cohort of ER+, HER2+ and HER2- patients undergoing concurrent FDG and FES PET scans, FDG and FES activity was similar regardless of HER2 status. FES uptake in both HER2- and HER2+ patients and stability over time in serial scans suggest that HER2 does not affect ER density. This suggests that in many patients with so-called “triple positive” disease, endocrine therapy may offer a powerful primary rather than ancillary tool in select patients. FES combined with FDG PET may offer utility in predicting and assessing response to therapy in this patient population. Table 1. FES and FDG uptake in patients with HER2- or HER2+ disease Citation Format: Natasha Hunter, Lanell M. Peterson, David A. Mankoff, Mark Muzi, Delphine Chen, William R. Gwin, Shaveta Vinayak, Nancy E Davidson, Jennifer M. Specht, Hannah Linden. Fluoroestradiol (FES) and Fluorodeoxyglucose (FDG) PET imaging in patients with ER+, HER2+ or HER2- metastatic breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-03-17.