Abstract PS11-03: Comparative analysis of [18F]FES- and [18F]FDG-PET in patients with metastatic ER+ lobular breast cancer

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Abstract BACKGROUND: Detection of invasive lobular carcinoma (ILC) with standard imaging modalities can be challenging. [18F]fluorodeoxyglucose (FDG) PET is commonly used in the work up of first presentation of metastatic breast cancer (MBC), but may be suboptimal for ILC due to lower metabolic activity compared to invasive ductal cancer. Imaging with [18F]fluoro-17b-estradiol (FES) PET measures whole-body estrogen receptor (ER) expression, which is generally high in ILC. Whether FES-PET outperforms FDG-PET in detecting metastatic ILC (mILC) lesions -and should therefore be the first-step imaging modality for mILC work up- remains unclear. Therefore, we compared the sensitivity of FDG- and FES-PET to detect ILC metastases, in patients who were enrolled in the prospective IMPACT-MBC study. METHODS: In IMPACT-MBC (NCT01957332), 200 patients with newly diagnosed MBC underwent baseline FDG-PET, FES-PET, computed tomography (CT) scan and bone scintigraphy in addition to a metastasis biopsy. In this subanalysis, patients with ER+ mILC were included. Metastases were visually identified on FDG- and FES-PET and expressed as the total number and percentage of all metastases detected on all imaging modalities together. Results were analyzed on patient and lesion level. RESULTS: Twenty-six patients with ER+ ILC were included, with a median age of 57 years. Mean number of detected metastases per patient was 34 (range 2-77) with FDG-PET and 33 (range 0-88) with FES-PET. In 12 patients (46%), FES-PET detected more metastases than FDG-PET, whereas FDG-PET detected more metastases than FES-PET in another 12 patients (46%), and in two patients (8%) the same number of metastases was detected with both scans. In total, 1101 metastases were detected with all imaging modalities. A total of 853 metastases were detected with FDG-PET and 859 metastases were detected with FES-PET. This results in a sensitivity of 77.5% (95% CI 75 – 80) for FDG-PET and 78% (95% CI 76 – 80) for FES-PET. CONCLUSIONS: This is the largest prospective cohort comparing FDG-PET with FES-PET for baseline work up in mILC. FES-PET does not outperform FDG-PET in detecting ILC metastases, and the two techniques have similar sensitivity on a group level. For clinical practice, in light of wider availability of FDG-PET and detection of equal or more ILC metastases in the majority of patients, FDG-PET remains the first-step imaging modality for mILC. However, in a considerable part of patients with mILC, FES-PET detects more ILC metastases than FDG-PET. Therefore, in case of persistent clinical dilemmas or doubts, an additional FES-PET should certainly be considered in mILC. Citation Format: Jasmine Moustaquim, Gabriëlle E. van Wonderen, Jasper J. L. van Geel, Michel van Kruchten, Erik F.J. de Vries, Agnes Jager, Evelien Kuip, Marcel P. M. Stokkel, Andor W.J.M. Glaudemans, Geke A. P. Hospers, C. Willemien Menke van der Houven van Oordt, Bert van der Vegt, Elisabeth G.E. de Vries, Carolina P. Schröder. Comparative analysis of [18F]FES- and [18F]FDG-PET in patients with metastatic ER+ lobular breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr PS11-03.

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