187 Denosumab reduces lesional Fluoride skeletal burden on Na[18F]F PET-CT ∆ =("#$#%&'&$!"##"$%&'( "#$#%&'&$()*+#,-+) "#$#%&'&$()*+#,-+ ∗ 100% In one patient (2 scans, non-denosumab subgroup), the measured bone turnover in FD did not exceed the cut-off for healthy bone, resulting in an FTV of 0 (zero) at baseline and follow-up and therefore DFTV was undefined. All variables were assessed for (log)normality based on skewness and kurtosis metrics. As all main parameters showed deviation of (log)normality, defined as 0 not being within the 95%-confidence interval of both distributionmetrics, these were described as median and interquartile range (IQR), non-parametric tests were used (Mann-Whitney U, Wilcoxon, Friedman’s ANOVA) and the non-parametric rank correlation coefficient Spearman’s ρ was used. In order to determine the intrapatient reproducibility of SUVcut-off between baseline and any of the follow-up scans, we computed the intraclass correlation coefficient (ICC), alpha two-way random model on absolute agreement and interpreted this value according to Koo, et al. [14]. Furthermore, reproducibility was graphically described using Bland-Altman plots including systematic error (mean absolute difference) and random error (95%-limits of agreement), which also visualizes heteroscedasticity. All statistical analyses were performed using SPSS software (version 26, IBM, New York, New York, USA). A type I error (p-value) below 0.05 was considered statistically significant. Results Fifteen patients met our eligibility criteria. Mean age at time of the baseline PET-CT was 47 years (range: 16 to 68 years), 9 (60%) were females [4]. The median interval between BTM-measurement and Na[18F]F PET-CT was 4 weeks (IQR 1 to 9 weeks, n=37 scans). This interval was significantly longer at baseline (median 24 days, biochemistry before PET-CT) compared to and follow-up (median 7 days, PET-CT before biochemistry) (n=15; p=0.006, Wilcoxon), however the absolute intervals between PET and serum biomarkers of baseline, first follow-up and second followup were not significantly different (n=6 triplets, p=0.568 Friedman’s ANOVA). This is in accordance with our standardized outpatient department procedure. Specific information for each patient on reason for treatment, treatment dosing and duration of treatment, the interval between Na[18F]F PET-CT and denosumab, interval between Na[18F]F PET-CT scans, interval between Na[18F]F PET-CT and BTMs, bases SBS and type of FD is summarized in table 1 and 2. 8