Is Response Assessment of Breast Cancer Bone Metastases Better with Measurement of (18)F-Fluoride Metabolic Flux Than with Measurement of (18)F-Fluoride PET/CT SUV?

Our purpose was to establish whether noninvasive measurement of changes in (18)F-fluoride metabolic flux to bone mineral (K(i)) by PET/CT can provide incremental value in response assessment of bone metastases in breast cancer compared with SUV(max) and SUV(mean). Methods: Twelve breast cancer patients starting endocrine treatment for de novo or progressive bone metastases were included. Static (18)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 (18)F-fluoride activity concentrations, and K(i) in individual bone metastases was calculated using a previously validated method. Percentage changes in K(i), SUV(max), and SUV(mean) 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. Results: Of the 4 patients with clinically progressive disease (PD), mean K(i) significantly increased (>25%) in all, SUV(max) in 3, and SUV(mean) in 2. Of the 8 non-PD patients, K(i) decreased or remained stable in 7, SUV(max) in 5, and SUV(mean) in 6. A significant mean percentage increase from baseline for K(i), compared with SUV(max) and SUV(mean), occurred in the 4 patients with PD (89.7% vs. 41.8% and 43.5%, respectively; P < 0.001). Conclusion: After 8 wk of endocrine treatment for bone-predominant metastatic breast cancer, K(i) more reliably differentiated PD from non-PD than did SUV(max) and SUV(mean), probably because measurement of SUV underestimates fluoride clearance by not considering changes in input function.