Changes in tophus composition during urate-lowering therapy: a dual energy CT study

The gouty tophus is an organized structure composed of monosodium urate (MSU) crystals and chronic inflammatory soft tissue. This dual energy CT (DECT) study aimed to determine whether the composition of the tophus changes during urate-lowering therapy.Serial DECT scans from 32 people with gout were obtained over two years of allopurinol therapy, dose-escalated to serum urate0.36mmol/L. Up to five index tophi were selected for each patient, with 103 separate tophi included in the analysis. Using manual outlining methods of conventional CT and DECT scans, the same index tophi were serially measured for total tophus volume and urate volume. For each tophus, the soft tissue volume was then calculated by subtracting the urate volume from the total tophus volume.The mean (SD) serum urate reduced from 0.43 (0.03) mmol/L at baseline to 0.31 (0.02) mmol/L at Year 2. The mean (SD) total tophus volume reduced over the two-year period; from 5.17 (5.55) cmThe composition of the tophus is dynamic and changes during urate-lowering therapy for gout management. The soft tissue component of the tophus is slower to respond and may persist without measurable MSU crystal deposition.