Abstract P1-08-29: Utility of 18F-FES PET/CT over 18F-FDG PET/CT in Initial Diagnosis of over Ninety Metastatic Lesions in a Patient with Metastatic ER+ Breast Cancer: A Case Report

Abstract Approximately 6% of women with newly diagnosed breast cancer will present with metastatic disease. Proper staging workup and diagnosis of metastatic lesions is crucial prior to surgical treatment. 18F-FDG PET/CT is often included in the staging workup of locally advanced breast cancer. Recently, PET/CT with 18F-Fluoroestradiol (18F-FES PET/CT), a radiolabeled form of estradiol that binds to the estrogen receptor, has been approved as an additional imaging technique for the detection of ER-positive lesions in patients with metastatic breast cancer. While the two have been shown to have comparable sensitivity for diagnosis of metastatic lesions, there is still much debate regarding when to use 18F-FES PET/CT over 18F-FDG PET/CT imaging. We present the case of a 68-year-old patient diagnosed with ER+/PR+ invasive ductal carcinoma of the left breast. On breast MRI, the patient was found to have a 1.3cm sternal mass as well as subtle enhancement of two right-sided ribs concerning for metastatic lesions. The patient first underwent a 18F-FDG PET/CT which did not reveal any metastatic lesions. Due to persistent concern for metastatic disease, the patient then underwent whole body 18F-FES PET/CT imaging. This demonstrated extensive osseous metastatic disease not visualized on the previous 18F-FDG PET/CT scan. There were over ninety foci of increased uptake, including multiple foci within the sternum (correlating to the lesion seen on MRI), as well as foci within the cervical vertebral bodies, ribs, sacrum and pelvis. The detection of multiple osseous metastatic lesions in our patient on 18F-FES PET/CT imaging demonstrates previous findings of increased specificity of 18F-FES PET/CT for ER+ metastatic lesions compared to 18F-FDG PET/CT imaging. In our patient, the results of her 18F-FES PET/CT resulted in upstaging of her cancer, thus preventing her from undergoing unnecessary surgical intervention. This likely resulted in earlier treatment with systemic chemotherapy as surgical intervention and resulting complications may delay initiation of chemotherapy. This study thus illustrates the potential value of 18F-FES PET/CT imaging in the diagnosis of ER+ metastatic lesions, especially in the setting of equivocal or discordant findings on conventional imaging. Citation Format: Caroline Breit, Valerie Gorman. Utility of 18F-FES PET/CT over 18F-FDG PET/CT in Initial Diagnosis of over Ninety Metastatic Lesions in a Patient with Metastatic ER+ Breast Cancer: A Case Report [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 P1-08-29.