Background/Purpose: Pegloticase is approved for lowering serum urate (sUA) in chronic refractory tophaceous gout (CRTG), but the rate of tophus resolution is not well defined, in part owing to limitations of measurement techniques. DECT permits 3-dimensional reconstruction and volumetric assessment of tophi, but comparisons between DECT and other imaging modalities are limited.  We conducted an n-of-1 pilot study comparing DECT, MSK-US and surface caliper measurement to assess the rate of tophus debulking during pegloticase treatment.

Methods: A 32-year old male with a 10 year history of CRTG underwent a 13-infusion, twice-monthly course of pegloticase 8 mg, with sUA assessment prior to each infusion. Monthly caliper measurements of 3 index tophi, photographs and MSK-US imaging (gray scale, 18 mHz probe) every 3 months, as well as DECT and X-Rays (pre-/post-treatment) of the hands and feet were performed and compared. 

Results: 8 tophi were noted on physical exam of the hands and feet, with 3 selected as index tophi.  Whereas physical exam identified 4 tophi on the right hand, DECT revealed 15; whereas X-ray identified a single erosive lesion in the right hand, DECT identified 4 more in the feet. Pegloticase persistently lowered the patient’s sUA to <0.5 mg/dL. Caliper measurements at the start and end of treatment revealed 73, 60, & 63% reductions of the index tophi, while MSK-US showed 29, 80, and 41% reductions of the same tophi, respectively. In contrast, DECT revealed 100% resolution of all 3 index tophi, and resolution/ improvement of all other tophi identified. Initial DECT images revealed a composite tophus volume of 2,170 mm³ on the right hand and 900mm³ on the left; each decreased to 130 mm³by study completion.  On caliper measurement, the size of the index tophi fluctuated over time despite overall reduction. On MSK-US, some individual tophi appeared to “soften” and expand initially, followed by overall size reduction.

Conclusion: Pegloticase rapidly reduced all tophi assessed.  While all assessment modalities were informative, correlation between them was poor, probably relating to the fact that calipers and MSK-US measure tophus area in non-congruent, perpendicular planes, whereas DECT measures volume. As previously reported, DECT identified occult urate deposition not visible on physical exam. Interestingly, tophi may fluctuate in size, even transiently increasing, during the process of resolution, revealed on ultrasound as tophus “softening” and loss of structure. Moreover, DECT imaging indicated that some urate deposits fully resolved even as their visible/palpable lesions persisted, possibly because of persistence of soft tissue swelling and/or fibrosis. We conclude that urate resorption begins early in the course of pegloticase therapy, but may be hard to recognize because of fluctuating volumes.  While all imaging modalities have value, DECT is superior for identifying total (including occult) urate deposition, assessing absolute volume of deposits, and confirming urate resolution, even when soft tissue swelling persists.   DECT may therefore be particularly valuable for determining the end point of pegloticase therapy.

Support:  Provided by Savient and Crealta

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/pegloticase-for-tophus-debulking-comparison-of-dual-energy-computerized-tomography-dect-musculoskeletal-ultrasound-msk-us-and-topographic-caliper-measurement-for-assessing-debulking-rate/