Abstract
OBJECTIVE: Previous studies reported low [18F] fluorodeoxyglucose ([18F]FDG) PET uptake in estrogen receptor-positive breast tumours, potentially missing detection of distant metastases. This study assessed the proportion of estrogen receptor-positive hypometabolic tumours, clinical factors influencing [18F]FDG uptake, and the prognostic impact. METHODS: Baseline [18F]FDG PET/computed tomography (CT) and [18F]FDG PET/MRI exams of female patients diagnosed with estrogen receptor-positive locally advanced (cT3-4N0 or cT1-4N+), metastatic, or recurrent breast cancer between 2013-2022 were retrospectively collected. Different thresholds of maximum standardised uptake value (SUVmax) and tumour-to-background ratio (TBR; SUVmax tumour/SUVmax background) were applied to determine the proportion of hypometabolic [18F]FDG PET exams. Logistic regression and survival analysis were performed. RESULTS: 119 patients underwent [18F]FDG PET/CT and 31 [18F]FDG PET/MRI. The proportion of hypometabolic tumours for SUVmax thresholds 2.0, 2.5, 3.0, TBR of contralateral breast less than or equal to 1, and TBR of liver less than or equal to 1 was 8.4, 15.1, 21.8, 5.1, and 28.6%, respectively for [18F]FDG PET/CT and 16.1, 19.4, 29.0, 6.9, and 35.5% for [18F]FDG PET/MRI. Clinically tumour status (cT-status), histology type, and tumour grade were associated with the presence of a hypometabolic tumour. No PET-derived variables were associated with recurrence-free survival. CONCLUSION: A considerable proportion of estrogen receptor-positive breast tumours showed low SUVmax, indicating potential suboptimal staging on [18F]FDG PET. In patients with lower cT-status, lobular histology and low-grade estrogen receptor-positive tumour, [18F]FDG PET may be less reliable as staging procedure. Further research is necessary to determine the optimal metabolic threshold for defining a hypometabolic tumour.