IMG-107.18F-fluoroestradiol PET/CT fused with MRI brain enhances detection and optimizes management of ER+ brain metastases

BACKGROUND MRI is the standard imaging modality for brain metastases (BrMs) evaluation; however, BrMs are difficult to characterize especially when prior treatments such as radiation therapy (RT) were administered. [18F]-fluoroestradiol (18F-FES) PET imaging can further characterize estrogen receptor positive (ER+) BrMs to inform management decisions. METHODS We identified ER+ breast cancer patients with BrMs who underwent 18F-FES PET between 2023-2024. Baseline characteristics were recorded. The FES-PET was fused with MRI brain and all lesions were reviewed. Discrepancies between PET and MRI findings were documented. The impact on clinical decision making was analyzed; each case was reviewed by a multidisciplinary team. RESULTS Twelve patients (77 BrMs) were included. In 8 patients with previous SRS, 18F-FES PET/MRI brain fusion distinguished between tumor recurrence vs radiation necrosis in 17 lesions. 5 lesions were deemed recurrent and retreated with GK SRS, resected (pathology-confirmed), or started on systemic therapy. All demonstrated improved radiographic, post-treatment imaging. Twelve non-avid lesions demonstrated clinical stability/improvement on subsequent MRI. Two patients had prior WBRT. The first had 26 lesions – 16 enhancing on MRI with 11 (69%) PET-avid. MRI demonstrated a mixed response; PET detected an additional 5 lesions. Clear progression was seen on follow-up MRI requiring repeat WBRT. The second had 31 lesions – 28 enhancing on MRI with 14 (50%) PET-avid. Systemic therapy was continued with stable, subsequent MRIs. MRI alone missed 10 lesions detected by 18F-FES PET in the cohort. Two patients had uncertain etiology; one confirmed meningioma and the other was suspected toxoplasmosis. Treatment was withheld due to non-avidity on 18F-FES PET. Management was modified for 11 patients (92%) following PET acquisition. CONCLUSIONS 18F-FES PET/MRI fusion offers valuable diagnostic advantages over MRI alone for detection of BrMs and clarification between residual tumor vs post-treatment changes. The results underscore the increased sensitivity and specificity of 18F-FES PET. Incorporating structural and molecular-based imaging enhances lesion detection and provides critical information that can impact clinical decision making.