Hybrid [¹⁸F] Fluoroestradiol Brain PET/CT/MRI for Detection of Estrogen Receptor–Positive Intracranial Metastases Compared with MRI and Vertex-to-Below the Knees PET Imaging

[18F]-fluoroestradiol (FES) is a novel molecular imaging radiopharmaceutical that has demonstrated promise in the staging of ER-positive brain metastases. Our aim was to determine if hybrid [18F] FES brain PET/MRI is superior to conventional brain MRI alone for detecting estrogen receptor (ER)-positive brain and calvarial metastases, compare limited [18F] FES brain PET with vertex-to-below the knees [18F] FES PET results, and determine the impact of [18F] FES brain PET/MRI on patient management decisions. Following institutional review board approval, a retrospective analysis was conducted of patients who underwent [18F] FES brain PET/CT and contrast-enhanced brain MRI at our institution from January 2024 to November 2024. Intracranial lesions, including parenchymal, dural-based, calvarial, leptomeningeal, and extracranial soft tissue/scalp, were analyzed by an expert neuroradiologist using fused [18F] FES brain PET/MRI, and lesion size, location, and maximum standard uptake value were recorded. Two radiology residents reviewed separately acquired contrast-enhanced brain MRI reports and documented the number and location of intracranial lesions. A nuclear medicine physician reviewed vertex-to-below the knees [18F] FES PET/CT and reported the intracranial lesion number and maximum standard uptake value. Ten female patients with ER-positive breast cancer with 90 metastatic brain and calvarial lesions were included in our analysis. The mean age was 64.3 (SD, 7.5) years. Twelve of 90 (13.3%) lesions were occult on contrast-enhanced brain MRI but detected by 18F-FES brain PET/MRI, including 1 parenchymal lesion, 8/19 (42.1%) calvarial lesions, and 5/6 (50.0%) extracranial soft-tissue/scalp lesions. Meanwhile, 16/90 (17.8%) lesions were occult on vertex-to-thigh 18F-FES PET/CT compared with [18F] FES brain PET/MRI, including 13/58 (22.4%) parenchymal lesions and 3/7 (42.9%) dural-based lesions. There was a modification in patient management for 9 of 10 patients (90%) following the [18F] FES brain PET/MRI. By integrating [18F] FES brain PET/CT/MRI into our clinical workflow, we improved the detection of ER-positive lesions, resulting in a substantial impact on clinical management decisions.