Differentiation of axial spondyloarthritis-related lesions from degenerative lesions on Na[18F]Fluoride PET/CT in a longitudinal dataset of patients with radiographic axial spondyloarthritis

Purpose To investigate longitudinal changes of Na[ 18 F]Fluoride uptake in the axial skeleton of patients with radiographic axial spondyloarthritis (r-axSpA) during biological treatment, focusing on r-axSpA and degenerative (D) lesions. Methods 20 active r-axSpA patients were included. Na[ 18 F]Fluoride Positron Emission Tomography / Computed Tomography (PET/CT) scans were performed prior to, as well as 12 and 52 weeks after starting biological therapy. PET-positive lesions in the axial skeleton were scored and classified by two blinded readers. Quantitative Na[ 18 F]Fluoride uptake was assessed by determination of standardized uptake values (SUV) corrected for individual integrated whole blood activity concentration (SUV AUC ). Results A total of 276 lesions with visual Na[ 18 F]Fluoride uptake were detected, of which 98 (36%) were defined as r-axSpA and 46 (17%) as D. The total number of r-axSpA lesions decreased from 98 at baseline to 67 at week 12 (-32%) and to 55 at week 52 (-18%), while the number of D lesions remained stable over time. The majority of r-axSpA lesions were observed in the thoracic spine (84%), while the majority of D lesions were observed in the lumbar spine (48%). Quantitatively, 84% and 88% of the r-axSpA lesions showed a decrease of Na[ 18 F]Fluoride uptake between baseline and 12 and 52 weeks, respectively. In contrast, most D lesions either showed an increase or mild decrease between baseline and follow-up. Conclusion Na[ 18 F]Fluoride uptake in visually classified r-axSpA and D lesions in r-axSpA patients on PET/CT demonstrated a striking different change in both the number of r-axSpA lesions and quantitative tracer uptake herein versus D lesions while on biological treatment.