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Large-size Intramuscular Nodular Fasciitis; A Challenging Histopathologic Diagnosis Confirmed by Molecular Detection of USP6 Gene Rearrangement: A Case Report and Literature Review

Changrong Wang, Wei Wang, Rujun Xu, Jingjing Xiang

The intramuscular subtype of Nodular Fasciitis (NF) is rare with lesions normally not more than 2 cm in size and characterized by pseudosarcomatous morphology. This paper presents the case of a 27-year-old man with a large-size intramuscular NF. The patient came for treatment complaining of an increasingly enlarged mass in the left upper arm for 4 months. Magnetic Resonance Imaging (MRI) confirmed the presence of a well-defined tumor measuring 5 cm within the outer edge of the middle humerus. Microscopically, the neoplasm was rich in fibroblasts and myofibroblasts in an interlaced pattern with high mitotic index and evident multinuclear giant cells. Erythrocyte extravasation was easily seen in the stroma. The tumor border was infiltrative. Immunohistochemically, the tumor cells were positive for Smooth Muscle Action (SMA) and negative for cytokeratin, desmin, CD34, S100, ALK, and β-catenin. Molecular detection demonstrated evidence of Ubiquitin-Specific Peptidase 6 (USP6) gene rearrangements in this tumor. Based on the findings, the tumor was diagnosed as intramuscular NF. At 56 months after the initial surgery, the patient had recovered with no evidence of recurrence or metastasis.