Tophaceous Gout Mimicking a Neoplasm of the Patella.

Tophaceous gout of the patella and extensor apparatus of the knee can mimic a musculoskeletal neoplasm. Considering gout, correlation with clinical history and presentation, and meticulous analysis of imaging semiology are key to a correct diagnosis. The most useful signs are periarticular erosion with overhanging edges, increased soft tissue on radiography and computed tomography, the "snowstorm sign" on ultrasound, variable signal and enhancement, bandlike infiltration of tendons, and osseous erosions with no or little bone marrow edema on magnetic resonance imaging. Dual-energy computed tomography is both a sensitive and specific tool for noninvasive characterization of gout and for differentiating other crystal arthropathies.