184 Part II Chapter 8 CT), Na[18F]F PET-CT has superior spatial resolution (~3.5 vs 8 mm FWHM), faster pharmacokinetics of the radiopharmaceutical (one hour or less vs four hours of incubation) and faster whole body 3-D imaging at the cost of higher expenses and lower availability [4]. We postulate that observed decreases in BTMs from described in our previous study [4] reflect a local response in Na[18F]F accumulation leading to a decrease in FTV and FAS and set out to evaluate this in patients with FD/MAS treated with denosumab. Materials and methods Patients were evaluated as part of an ongoing observational study. Denosumab and bisphosphonates were prescribed off-label on clinical grounds. Responses to these therapies were then analyzed retrospectively. Population Written informed consent was obtained fromall patients and the study was approved by the medical ethical committee of the LUMC. All FD/MAS patients between February 2015 and April 2018 before start of denosumab treatment or planned surgery were included. They all underwent at least two consecutive full body Na[18F]F PET-CT scans, the first performed for baseline assessment of FD disease burden at start of denosumab or under bisphosphonates (=non-denosumab) and at least one follow-up Na[18F]F PET-CT scan, between February 2015 and August 2019 (n=15). These patients are a subgroup of our previously reported study on Na[18F]F PET-CT in FD/MAS [4]. Reasons for not having follow-up Na[18F]F PET-CT scans (n=5) were of medically unrelated logistic nature (the scans were performed at another location), or because the second PET could not be planned within a reasonable time-window or therapy was not started at all. Treatment goals The decision to start off-label denosumab was based on clinical or biochemical failure of previous bisphosphonate treatment or biochemically progressive disease despite prior treatment with bisphosphonates (mainly olpadronate). However, after full consultation, in some patients surgery for the clinically most relevant lesion was considered for a time point after the window of this study or patients refused denosumab and therefore stayed on-treatment with bisphosphonates. The goal of their treatment was decreasing pain measured by the Brief Pain Inventory (BPI), improvement of their clinical status (patients’ own report) and decrease of