Thymic CT attenuation score in SAPHO syndrome: a comparative study with rheumatoid arthritis, ankylosing spondylitis, and healthy controls

Objective SAPHO syndrome is a rare autoinflammatory/autoimmune overlap disorder whose thymic imaging characteristics remain unexplored. This study evaluated thymic CT attenuation scores in patients with SAPHO syndrome compared with rheumatoid arthritis (RA), ankylosing spondylitis (AS), and healthy controls (HC). Methods This retrospective study included 180 patients with SAPHO syndrome, 60 with RA, 60 with AS, and 180 HC, all aged ≥30 years. Thymic attenuation was assessed using a 4-point scale (0–3) and CT attenuation values in Hounsfield units (HU). Higher attenuation was defined as score ≥2. Subgroup analyses were performed by sex and age (<45 vs ≥45 years). Results The prevalence of higher thymic attenuation differed significantly among the four groups (P<0.001), with proportions of 31.1% in SAPHO, 25.0% in RA, 8.3% in AS, and 10.6% in HC. Post hoc analyses of the ordinal attenuation score and CT attenuation values showed higher values in SAPHO and RA than in AS and HC, whereas SAPHO and RA did not differ significantly from each other. This pattern appeared most pronounced among women aged <45 years, in whom higher attenuation was observed in 54.3% of SAPHO and 47.8% of RA patients, compared with 19.2% of HC and 3.8% of AS patients. Within the SAPHO group, higher attenuation was associated with female sex (85.7% vs 68.5%, P = 0.015) and younger age (median 37.5 vs 44 years, P<0.001). Conclusion Thymic CT attenuation was higher in SAPHO and RA, particularly among women aged <45 years, whereas AS showed findings similar to HC. Increased thymic attenuation may represent a potential imaging phenotype associated with SAPHO syndrome and RA. Further longitudinal and pathological studies are needed to clarify its biological significance.