The proportion, clinical predictors, and prognostic impact of hypometabolic estrogen receptor–positive primary breast cancer on baseline [18F] fluorodeoxyglucose PET

OBJECTIVE: Previous studies reported low [ 18 F] fluorodeoxyglucose ([ 18 F]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 [ 18 F]FDG uptake, and the prognostic impact. METHODS: Baseline [ 18 F]FDG PET/computed tomography (CT) and [ 18 F]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 (SUV max ) and tumour-to-background ratio (TBR; SUV max tumour/SUV max background) were applied to determine the proportion of hypometabolic [ 18 F]FDG PET exams. Logistic regression and survival analysis were performed. RESULTS: 119 patients underwent [ 18 F]FDG PET/CT and 31 [ 18 F]FDG PET/MRI. The proportion of hypometabolic tumours for SUV max 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 [ 18 F]FDG PET/CT and 16.1, 19.4, 29.0, 6.9, and 35.5% for [ 18 F]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 SUV max , indicating potential suboptimal staging on [ 18 F]FDG PET. In patients with lower cT-status, lobular histology and low-grade estrogen receptor-positive tumour, [ 18 F]FDG PET may be less reliable as staging procedure. Further research is necessary to determine the optimal metabolic threshold for defining a hypometabolic tumour.