Intraarticular Nodular Fasciitis of the Elbow Confirmed by USP6-MYH9 Gene Fusion.

Abstract Background: Nodular fasciitis is a benign, usually self-limiting myofibroblastic proliferation with a predilection for the upper extremities, trunk, and head and neck of young adults. Although it usually arises within subcutaneous tissues, it may also occur within skeletal muscle, dermis, vessels, and peripheral nerves. Since nodular fasciitis is not widely recognized to arise within joints, it may therefore cause diagnostic confusion in this uncommon setting. Histology can vary considerably and can, therefore, mimic other benign or low grade malignant neoplasms. Recent molecular analysis revealed that most cases of nodular fasciitis involve USP6-MYH9fusion. Case presentation: We report an unusual case of an 11-year-old patient who presented with a 6-month history of right elbow swelling and pain. Radiological examinations suggested tenosynovial giant cell tumor. 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 rearrangementinvolving USP6 and MYH9, supporting the diagnosis of nodular fasciitis within the elbow joint. Conclusion: This is the first case of intraarticular nodular fasciitis within the elbow joint in which MYH9-USP6 gene fusion was identified by molecular analysis. Nodular fasciitis should be considered in the differential diagnosis of intraarticular lesions.