Changes in Tophus Composition During Urate-Lowering Therapy: A Dual-Energy Computed Tomography Study.

Objective: The gouty tophus is an organized structure composed of monosodium urate (MSU) crystals and chronic inflammatory soft tissue. This dual-energy computed tomography (DECT) study aimed to determine whether the composition of the tophus changes during urate-lowering therapy.

Methods: Serial DECT scans from 32 people with gout were obtained over 2 years of allopurinol therapy, dose-escalated to serum urate of <0.36 mmoles/liter. Up to 5 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.

Results: The mean ± SD serum urate reduced from 0.43 ± 0.03 mmoles/liter at baseline to 0.31 ± 0.02 mmoles/liter at year 2. The mean ± SD total tophus volume reduced over the 2-year period from 5.17 ± 5.55 cm to 2.61 ± 2.73 cm (P < 0.0001). Greater reductions in tophus urate volumes than tophus soft tissue volumes were observed; the tophus urate volume decreased by 70.6%, and tophus soft tissue volume decreased by 37.8% (P < 0.0001). The mean tophus urate:soft tissue ratio reduced from 0.15 at baseline to 0.05 at year 2 (P < 0.001).

Conclusion: The 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.