[18F]FES PET/CT for the Detection of Distant Metastasis in Low-grade ER-positive Stage III Breast Cancer

This study aims to evaluate the detection rate of distant metastases using \[18F\]FES PET/CT in patients with low-grade, ER-positive stage III breast cancer.

Breast cancer is highly prevalent, and approximately 75% of cases are estrogen receptor (ER)-positive. Stage III breast cancer carries a significant risk of distant metastasis; therefore, NCCN and ESMO guidelines recommend systemic staging with CT, bone scan, or 18F-FDG PET/CT. However, 18F-FDG PET/CT demonstrates limited sensitivity in low-grade ER-positive tumors and invasive lobular carcinoma (ILC), and can yield false positives due to inflammatory reaction or reactive hyperplasia. Thus, more accurate imaging modalities are needed in this subgroup. 18F-fluoroestradiol (18F-FES) PET/CT visualizes ER expression and has shown high concordance with histopathology. A phase III study demonstrated 77% sensitivity and 100% specificity for metastatic disease. Emerging evidence suggests that 18F-FES PET/CT may detect distant or regional lymph node metastases more accurately than standard imaging, particularly in ILC and low-grade tumors, and can influence management decisions such as surgery and radiotherapy planning. Accurate identification of regional nodal involvement (axillary, supraclavicular, internal mammary nodes) is essential for determining the extent of axillary surgery after neoadjuvant chemotherapy and for tailoring regional nodal irradiation. Prior studies show that metabolic imaging can alter radiotherapy fields in up to 40% of patients. The primary objective is to determine the patient-based detection rate of qualitative 18F-FES PET/CT for distant metastases. Secondary objectives include evaluating its patient-based sensitivity and specificity for distant metastases; determining the detection rate, sensitivity, and specificity for cN3 lymph node metastases (infraclavicular, supraclavicular, and internal mammary nodes); assessing the rate of management change attributable to 18F-FES PET/CT; and comparing 18F-FES PET/CT with 18F-FDG PET/CT for detection rate, diagnostic accuracy, and management impact. Exploratory analyses will examine the correlation between qualitative and quantitative 18F-FES PET/CT findings and pathological response to neoadjuvant chemotherapy.