185 Denosumab reduces lesional Fluoride skeletal burden on Na[18F]F PET-CT painkiller use, which is always the treatment goal in our outpatient clinic, also with bisphosphonates [4, 13]. For these patients a baseline scan (n=15), first follow-up after start of treatment (n=15), second (n=6) and third (n=1) follow-up were available. After baseline Na[18F]F PET-CT, denosumab 60mg (Prolia®, Amgen Europe B.V.) trimonthly subcutaneous injections were started, in 8 of the 15 patients [4]. One patient started denosumab treatment later on because of worsening of inflammatory bowel disease. All patients starting denosumab, were previously treated with bisphosphonates. Thirteen patients (87%) used bisphosphonates at baseline Na[18F]F PET-CT, primarily olpadronate [(3-dimethylamino-1-hydroxypropylidene)-1,1 bisphosphonate], as reported before [4]. The non-denosumab group (n=7) was more heterogeneous: bisphosphonates were commenced (n=2), continued (n=3), or ceased before the period of our investigation (n=2). We used the 60 mg dose which is titrated in relation to complaints and biochemical response and varies per patient. The minimum maintenance dose is 60 mg twice a year (osteoporosis dose). In Table 1 and 2, we report on the specific dosing of denosumab and bisphosphonates including the duration of treatment and the relation to the date of Na[18F]F PET-CT and BTMs. Co-treatment with calcium/ D3/active vitamin Dmetabolites was initiated at discretion of the treating physician. Eleven patients (73%) were diagnosed with polyostotic FD of whom 2 were diagnosed with MAS, all with well-controlled endocrinopathies. The remaining 4 patients (27%) were diagnosed with monostotic FD. Reasons for repeated scans were to observe if the improvements clinically and biochemically also reflected changes on a local level. If the activity indeed went down, we gradually increased the dosing interval of the denosumab to observe if we could even gradually taper off or treat with the ‘osteoporosis dose’ to minimize exposure. We aimed to perform new scans at a 18 months-24 months interval (after 4-6 injections). The following clinical items were collected from the electronic patient files: FD localizations, demographic features, previous and current drug use. Non-fasting morning venous blood samples were collected for evaluation of bone and mineral metabolism (ALP and P1NP) and data on change in pain complaints using the brief pain inventory (BPI) were registered around the time of the Na[18F]F PET-CT [4]. A follow-up Na[18F]F PET-CT was scheduled approximately 1.5 years after baseline investigation and was accompanied by an outpatient clinic visit and laboratory test. Serum ALP activity was measured using a fully automated P800 modulator system and P1NP was determined by the E-170 system (both: Roche BV, Woerden, the Netherlands). The upper limit of normal values for these BTMs were 98 IU/L for 8