193 Denosumab reduces lesional Fluoride skeletal burden on Na[18F]F PET-CT Quantification of FD burden on Na[18F]F PET-CT at baseline and at first follow-up Median (IQR) change in FTV between baseline and at first follow-up for relevant subgroups is displayed in Table 4. Table 4. Median (IQR) change in FTV for relevant subgroups Group (n) FTV (cm3) Median (IQR) ∆FTV (%) Median (IQR) p-value ** Wilcoxon SUVpeak (g/mL) Median (IQR) p-value ** Wilcoxon Baseline First follow-up Baseline First follow-up Total (15 pairs) 199 (77-923) 97 (51-635) -42 (-13 - -66)* 0.030 29 (18-34) 21 (13-30) 0.028 Completed denosumab (7 pairs)# 361 (122-1,247) 97 (56-684) -61 (-71 - -21) 0.018 29 (18-34) 26 (13-30) 0.176 nondenosumab (7 pairs) 76 (13-814) 54 (8-253) -23 (-69 - +4)* 0.249 26 (11-32) 21 (11-23) 0.028 * for one patient in the non-denosumab subgroup ∆FTV was undefined as FD bone turnover did not exceed the cut-off for normal bone at baseline and during follow-up (i.e. FTV=0). ** significant findings depicted in bold. # one planned denosumab patient could not complete denosumab treatment because of comorbidity (IBD) and was removed from this analysis. This patient was included in the analysis of the group ‘Total’ to reflect the effect in all analyzed patients. The portion of baseline affected skeleton on Na[18F]F PET-CT (FAS) showed clearly stronger correlation with the current standard SBS (representing the segmented fraction of the affected skeleton on scintigraphy) (p=0.052) than with measured pathological Na[18F]F-volume (FTV, p=0.221). Although median SUVpeak at baseline did decrease significantly in the whole group after bone remodeling therapy (p=0.028), SUVpeak nonetheless did not change significantly in the subgroups of denosumab (p=0.176; n=7 pairs) and non-denosumab (p=0.080; n=7 pairs). For the 12 patients that completed treatment (as described in the Materials section), an even stronger FTV-decrease was measured: median (IQR) 158 cm3 (77-754 cm3) to 82 cm3 (45-248 cm3), p=0.003, median ∆FTV -59% (-21% to -69%). In one patient of the denosumab group, FTV initially significantly increased from 204 cm3 to 618 cm3, which was in accordance to biochemical parameters with ALP increasing from 189 to 253 IU/L and P1NP from 396 to 473 ng/mL. This was 8