Background: The histology and pattern of spread in lobular breast cancer has presented challenges in estimating extent of disease by traditional imaging methods. 18F-FES is an estrogen analogue PET imaging tracer which measures tumor ER expression at multiple tumor sites simultaneously. It is FDA approved in the US and will be available in 2021, pressing clinicians to define the role of FES in patients with breast cancer before use in practice. We compared quantitative FES-PET and clinical FDG-PET SUV uptake between patients with metastatic lobular carcinoma.

Methods: Metastatic lesions in patients with primary ER+ lobular breast cancer were retrospectively evaluated with FES- and FDG-PET. SUV uptake was calculated in 38 patients enrolled in various studies at our institution. Up to ten matched lesions in each patient were assessed for a total of 192 lesions. Pairwise t-tests were performed to measure the difference between paired FES and FDG lesions.

Results: Among metastatic breast cancer patients with lobular histology, 87% of patients had a positive FES scan. The majority of positive scans demonstrated uptake in all sites, with only a small percentage (6%) lacking FES uptake in at least one lesion. Uptake was noted in various metastatic tumor sites including bone (78%), soft tissue/lymph node (17%), breast (8%) and lung (1%). Mean (range) SUVmax in FES and FDG respectively was 3.38 (0.88, 9.08) and 4.14 (1.25, 9.49). Paired lesion analysis showed concordance between the two imaging modalities in lesions with low SUV uptake. FES and FDG uptake, however, could be discordant in both directions (FES > FDG, and FDG > FES) when there is higher level of uptake in at least one of the tracers. This was specifically seen in the 150 bone lesions, possibly indicating differences in lesion phenotype or response to therapy.

Conclusions: FES and FDG scans demonstrate similar average SUV uptake in metastatic lobular breast cancer, suggesting that both scans have utility in detecting lobular histology. Our data suggests that at higher SUV values, FES may provide additional information to FDG. Discordance between FES and FDG SUV uptake in the same lesions, especially in bone, has important implications for staging and assessing response to therapy. Large prospective trials are needed to define the clinical utility of FES-PET in metastatic lobular breast cancer.

Research Support: P01CA42045, R01CA72064, U01CA148131, DOD W81XWH-04-01-0675

Citation Format: Poorni Manohar, Lanell Peterson, Vicky Wu, Isaac Jenkins, Mark Muzi, Jennifer Specht, Delphine Chen, Jeanne Link, Kenneth Krohn, Paul Kinahan, David Mankoff, Hannah Linden. 18F-fluoroestradiol (fes) and 18f-fluorodeoxyglucose (fdg) pet imaging in metastatic lobular breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD6-10.