Diagnostic Performance of 18F-Fluoroestradiol PET/CT for Axillary Lymph Node Metastasis in Invasive Lobular Carcinoma: A Prospective Feasibility Study

Abstract

Invasive lobular breast cancer (ILC) presents diagnostic challenges with conventional imaging modalities. We aimed to evaluate the diagnostic performance of ¹⁸F-fluoroestradiol (¹⁸F-FES) PET/CT for axillary lymph node (ALN) metastasis in ILC. Methods: This prospective feasibility study was conducted between August 2023 and August 2024. Eligible patients had newly diagnosed estrogen receptor–positive ILC, had confirmed or suspected ALN metastasis on physical examination or imaging, and were scheduled to undergo axillary surgery. The presence of metastasis in ipsilateral ALN on ¹⁸F-FES PET/CT was assessed visually and semiquantitatively (SUV_max) and compared with surgical specimens. Results: In total, 20 women were included in the analysis. Twelve patients had surgically proven ALN metastasis. On visual analysis, ¹⁸F-FES PET/CT showed a sensitivity of 67% (8/12; 95% CI, 35%–90%) and a specificity of 100% (8/8; 95% CI, 63%–100%). All 4 false-negative cases had a single metastasis on the sentinel lymph node, with sizes of 1, 1.5, 6, and 6 mm. An unexpected ¹⁸F-FES–positive ipsilateral internal mammary lymph node was detected in 1 patient. The median SUV_max was 3.4 (interquartile range, 1.2–9.5) and 1.0 (interquartile range, 0.8–1.1) for metastatic and benign ALNs, respectively. With an optimal cutoff of at least 1.2, SUV_max showed a sensitivity of 75% (9/12; 95% CI, 43%–95%) and a specificity of 100% (8/8; 95% CI, 63%–100%). The area under the receiver operating curve was 0.89 (95% CI, 0.75–1.00). Three of 4 patients with false-negative fine-needle aspiration results for ALN were positive on ¹⁸F-FES PET/CT. Conclusion: ¹⁸F-FES PET/CT had excellent specificity and moderate sensitivity for detecting ALN metastasis in ILC, suggesting its potential as a valuable staging modality for early ILC.