Is Response Assessment of Breast Cancer Bone Metastases Better with Measurement of F-Fluoride Metabolic Flux Than with Measurement of F-Fluoride PET/CT SUV?

Our purpose was to establish whether noninvasive measurement of changes in F-fluoride metabolic flux to bone mineral (K) by PET/CT can provide incremental value in response assessment of bone metastases in breast cancer compared with SUV and SUV Twelve breast cancer patients starting endocrine treatment for de novo or progressive bone metastases were included. Static F-fluoride PET/CT scans were acquired 60 min after injection, before and 8 wk after commencing treatment. Venous blood samples were taken at 55 and 85 min after injection to measure plasma F-fluoride activity concentrations, and K in individual bone metastases was calculated using a previously validated method. Percentage changes in K, SUV, and SUV were calculated from the same index lesions (≤5 lesions) from each patient. Clinical response up to 24 wk, assessed in consensus by 2 experienced oncologists masked to PET imaging findings, was used as a reference standard. Of the 4 patients with clinically progressive disease (PD), mean K significantly increased (>25%) in all, SUV in 3, and SUV in 2. Of the 8 non-PD patients, K decreased or remained stable in 7, SUV in 5, and SUV in 6. A significant mean percentage increase from baseline for K, compared with SUV and SUV, occurred in the 4 patients with PD (89.7% vs. 41.8% and 43.5%, respectively; < 0.001). After 8 wk of endocrine treatment for bone-predominant metastatic breast cancer, K more reliably differentiated PD from non-PD than did SUV and SUV, probably because measurement of SUV underestimates fluoride clearance by not considering changes in input function.