Intraarticular Nodular Fasciitis of the Elbow Confirmed by <i>UPS6::MYH9</i> Gene Fusion

Nodular fasciitis is a benign, usually self-limiting myofibroblastic proliferation with a predilection for the upper extremities, trunk, and head and neck, and almost all of which harbor the <i>USP6::MYH9</i> fusion. Since nodular fasciitis is not widely recognized to arise within the joints, it may therefore cause diagnostic confusion in this uncommon setting. We report an unusual tumor of an 11-year-old patient who presented with a 6-month history of right elbow swelling and pain. Histology revealed that the tumor was composed solely of proliferating bland spindle cells with osteoclast-like giant cells and scattered foam cells. There was only a focal area of immature granulation-like myxoid change, which suggested nodular fasciitis. Molecular analysis using next-generation sequencing revealed gene rearrangement involving <i>USP6</i> and <i>MYH9</i>, supporting the diagnosis of nodular fasciitis within the elbow joint. This is the first of intraarticular nodular fasciitis within the elbow joint in which <i>USP6::MYH9</i> gene fusion was identified by molecular analysis.