Intraarticular nodular fasciitis of the knee with MHY9-USP6 fusion

Kurzzusammenfassung Nodular fasciitis (NF) is a self-limiting, benign mesenchymal neoplasm of fibroblastic/myofibroblastic origin. Due to its fast growth, cellularity and frequently observed high mitotic count, it is commonly misdiagnosed as a malignant tumor, often resulting in overtreatment. Intraarticular examples of NF are rare. Radiologically, it can mimic fibroma of the tendon sheath, tenosynovial giant cell tumor or synovial chondromatosis. Recently, MYH9-USP6 fusion has been found in up to 92% of NF’s. Herein, we report a case of a 38-year-old patient with an intraarticular lesion, radiologically suspicious of tenosynovial giant cell tumor. Histology demonstrated a spindle cell lesion composed of fibroblasts/myofibroblasts embedded in a highly collagenous/hyalinized stroma, partly arranged in short fascicles. Extravasated erythrocytes and rare mitotic figures were present. Immunohistochemically, tumor cells expressed SMA whereas the cells were negative for desmin, β-catenin, CD34, and SOX10. Molecular analysis using NGS (Archer Fusion Plex Sarcoma Panel) revealed gene rearrangement involving USP6 and MYH9 supporting the diagnosis of nodular fasciitis in the knee joint.

Lernziele Radiological and histological features of NF can overlap with other benign and low-grade malignant lesions. Identification of the USP6 gene rearrangements or finding of the MYH9-USP6 fusion especially in core needle biopsies and in lesions occurring at unusual sites can result in an adequate therapeutic approach avoiding overtreatment.