Intraarticular Nodular Fasciitis of the Elbow Confirmed by UPS6::MYH9 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 USP6::MYH9 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 USP6 and MYH9, supporting the diagnosis of nodular fasciitis within the elbow joint. This is the first of intraarticular nodular fasciitis within the elbow joint in which USP6::MYH9 gene fusion was identified by molecular analysis.