182 Part II Chapter 8 Abstract Context The correlation between Fibrous Dysplasia/McCune-Albright syndrome (FD/ MAS) skeletal disease burden on Na[18F]F-PET-CT and serum bone turnover markers (BTMs) was recently described. The effect of treatment on lesional fluoride burden in FD/MAS is unknown. Objective To investigate treatment response measurements in FD/MAS patients who underwent Na[18F]F-PET-CT and treatment with antiresorptives. Design Observational case series. Setting Academic center of expertise for rare bone diseases. Patients 15 consecutive patients with FD/MAS with baseline and follow-up Na[18F]F- PET-CT-parameters of healthy bone and FD lesions, BTMs and pain scores at start of denosumab (n=8) and non-denosumab patients (n=7). Main outcome(s) On Na[18F]F-PET-CT the Volumetric measures of FD-burden (FTV) and ‘Fraction affected skeleton’ (FAS), represented the portion of the skeleton affected. This was correlated with BTMs and pain. Results Disease activity decreased significantly, with FTV 361cm3 to 97cm3, p=0.018 in denosumab-treated patients, not in non-denosumab patients (p=0.249). Serum P1NP and Alkaline Phosphatase (ALP) decreased significantly: 82ng/mL vs 55ng/mL, p=0.023 and 119 IU/L vs 84 IU/L, p=0.020, respectively. In denosumab-treated patients pain scores improved leading to pain medication reduction. This correlated with lesional uptake whilst healthy bone activity did not change. BTMs and FTV correlated positively, P1NP r=0.730; p<0.001 and ALP r=0.406; p=0.006, as dis change in BTMs and FTV: P1NP (p=0.032) and ALP (p=0.024). FAS strongly correlated with treatmentinduced decrease in ALP (p=0.027) and P1NP (p=0.009). Conclusion Na[18F]F-PET-CT captured treatment-induced lesional changes which correlated with BTMs and pain reduction. Therefore Na[18F]F-PET-CT can be used as an objective local parameter of response to denosumab treatment in FD/MAS.