Contribution of bone scintigraphy and PET/CT in the differential diagnosis of bone involvement in a patient with fibrous dysplasia and monoclonal gammopathy of undetermined significance and fibrous dysplasia treatment with denosumab.

We describe a patient who has both fibrous dysplasia (FD) and monoclonal gammopathy of undetermined significance (MGUS). The simultaneous occurrence of these two pathological conditions is rare, and it is necessary to distinguish whether the bones are being affected by FD or multiple myeloma. The patient's problems began with a non-healing tooth extraction, raising suspicion of osteomyelitis. Later, he was also examined for fever of unknown origin, as he had elevated inflammatory markers, low-grade fever, and headaches. According to skeletal scintigraphy, the patient had a craniofacial skeleton affected by FD. Given the presence of IgG lambda type MGUS, it was necessary to rule out transformation of MGUS into multiple myeloma. PET/CT examination with the radiopharmaceuticals sodium fluoride (18F-NaF) and fluorodeoxyglucose (18F-FDG) was used to differentiate this. In the craniofacial skeleton area, the CT image corresponded to FD, and these lesions showed significant NaF accumulation. 18F-FDG accumulation was less pronounced, and none of the lesions with a high 18F-NaF uptake had an osteolytic character, which is indicative of skeletal involvement by FD rather than multiple myeloma. The patient was treated with denosumab, with an initial administration at 28-day intervals and a planned extension of the intervals to 3 months after stabilization of the condition. Immediately after the first denosumab injection, the headaches caused by craniofacial skeletal involvement due to FD disappeared. In the coexistence of MGUS and FD, it is necessary to precisely determine whether the bones are affected by FD or multiple myeloma by using PET/CT imaging and radiopharmaceuticals 18F-NaF and 18F-FDG. The text provides an overview of the drug treatment of FD from a 2026 perspective.