Variation in urinary excretion of FDG, yet another uncertainty in quantitative PET

BACKGROUND The standardized uptake value (SUV) is the most common estimate of metabolic activity used in clinical positron emission tomography (PET). Several biological and technological factors influence the accurate SUV calculation. PURPOSE To assess another potential source of variability of the SUV, the variations in urinary excretion of fluorodeoxyglucose (FDG). MATERIAL AND METHODS Twenty patients with various malignancies scheduled for PET/CT with 18F-FDG were included in the present study. The activity in urine voided immediately before image acquisition was measured and decay corrected. An estimation of FDG content in the urinary bladder was made during imaging, and the two components of urinary FDG were added. The urinary output of FDG, and the quantity of FDG divided by the time to measurements, was estimated. RESULTS The excretion of FDG in urine was between 5.7% and 15.2% of injected dose (decay corrected), and from 0.06% to 0.3%/min after injection, a five-fold difference in clearance. CONCLUSION About 10% of injected dose is excreted in urine at 70 min post injection, but the urinary FDG excretion was found to be highly variable, yet another uncertainty affecting the SUV measurements.