Although the precise pathological mechanisms underlying the features of the RA synovium observed on 18F FDG-PET have not been fully elucidated, a significant decrease in the serum MMP-3 level, which is thought to primarily reflect the volume of the RA synovium, was observed at both three and six months after the initiation of TCZ treatment in the present study. Here, MMP3 is linked to rheumatoid arthritis.