Articular nodular fasciitis of the right shoulder joint: report of an unusual case with focus on immunohistochemical differential diagnosis.

The mesenchymal lesion nodular fasciitis (NF) can affect various sites of the body but usually arises in subcutaneous tissue or occasionally skeletal muscle. NF is not commonly known to arise in joints, and articular NF is extremely rare. Herein, we present a case of a 54-year-old woman with articular NF. No sign of recurrence was observed after surgical piecemeal removal with a suspected positive surgical margin. In our case, a differential diagnosis of NF, desmoid-type fibromatosis, and low-grade myofibroblastic sarcoma was considered. Stromal hyalinization, a characteristic of articular NF, made the diagnosis somewhat difficult, although typical NF morphology was present. Immunohistochemical analysis of α-smooth muscle actin, desmin, β-catenin, and protein gene product 9.5 expression along with close morphological examination provided a reliable distinction.