F- FDG PET/CT joint assessment of early therapeutic response in rheumatoid arthritis patients treated with rituximab.

Background: F-FDG PET/CT has been proposed in the evaluation of the disease activity in rheumatoid arthritis (RA). The goals of this study were to evaluate the reproducibility of the technique, to compare metabolic parameters to clinical, biological and ultrasonographic parameters before and after treatment and to evaluate whether the early metabolic response was related to the outcome. F- FDG PET/CT of the hands, wrists and knees was obtained in 15 patients with anti-TNFα refractory RA, at baseline and 16 weeks after treatment with rituximab. The number of PET-positive joints (PET+ joints), the cumulative standard uptake value (cSUV) and the composite index (CI) were defined. The composite clinical index DAS, CRP serum levels and the number of joints positive at ultrasonography (US+ joints) and the cumulative synovial thickness (CST) were also assessed at baseline and week 24.

Results: High interobserver agreement was observed, both at baseline and after treatment. The number of PET+ joints was not correlated with the number of joints tender or swollen. The 3 metabolic parameters were strongly correlated with US, CRP and DAS at baseline and with US and CRP (CSUV, CI) at week 16, but no longer with the DAS index. The metabolic response based on the change in the visual PET/CT joint analysis predicted the outcome with a high negative predictive value of 91%, with a 91% specificity, and an 86% accuracy.

Conclusions: These preliminary data suggest that F- FDG PET/CT is a reproducible and accurate tool for evaluating disease activity in refractory rheumatoid arthritis and its non-response to rituximab. The correlation obtained with US joint assessment gives relevance to objective diseased joints through imaging techniques.