Nodular Fasciitis of the Piriformis Muscle Initially Concerning for Metastasis in a Patient With Previous History of Colon Adenocarcinoma: A Case Report

Case We report a case of nodular fasciitis (NF) of the hip in a patient with a history of colon adenocarcinoma, concerning for metastasis. PET/CT showed a hypermetabolic lesion, and MRI revealed a well‐defined, enhancing intramuscular mass. Histology identified fascicles of spindle or stellate cells with ovoid nuclei in myxoid and collagenous stroma with sparse inflammatory cells. On immunohistochemistry, the spindle cells expressed smooth muscle actin. Fluorescence in situ hybridization (FISH) demonstrated ubiquitin‐specific protease 6 (USP‐6) gene rearrangement, confirming the diagnosis. Conclusion NF is a benign, rapidly growing soft tissue lesion mimicking malignancy. Surgical excision is curative. Accurate diagnosis can prevent unnecessary interventions.