161 Quantifying skeletal burden in fibrous dysplasia using Sodium Fluoride PET/CT Quantification of FD burden on Na[18F]F-PET/CT The correlation between SUVmax and SUVpeak was very high (R2=0.956, p<0.001; Spearman), as was the correlation between TLF and FTV (R2=0.986, p<0.001; Spearman), therefore by predefinition, SUVmax and FTV were dropped from further analyses. SUVpeak did not correlate with SBS (R2=0.201, p=0.198; Spearman) and there was a significant but weak correlation between TLF and SBS (R2=0.384, p=0.047; Spearman). Still, in some of the included patients there was a discrepancy between TLF and SBS (Figure B). FD-burden quantification on Na[18F]F-PET/CT reached near-perfect interobserver agreement for both SUVpeak (ICC=0.994, p<0.001) and TLF (ICC=0.999, p<0.001) both with narrow limits of agreement (mean difference: -0.03 g/mL, limits of agreement: -3.4 to +3.3 g/mL for SUVpeak, and mean difference: -151 g, limits of agreement: -1084 to +781 g for TLF) (figure C). Scoring was feasible, with Na[18F]F-PET/CT taking about 10 minutes per patient and SBS on planar bone scintigraphy took about 5 minutes per patient. Patient 6 Patient 7 6