Background: The intramuscular subtype of nodular fasciitis (NF) is rare with lesions normally not more than 2 cm in size and characterized by pseudosarcomatous morphology.
Case presentation: Here, we report a case of a 27-year-old man presenting with a chief complaint of an increasingly enlarged mass of the left upper arm for 4 months. Magnetic resonance imaging confirmed the presence of a well-defined tumor measuring 5 cm within the outer edge of the middle humerus. Microscopically, the neoplasm was highly cellular with focally discohesive and myxoid features. In more cellular areas, cells grew in bundles with an interlaced pattern. The mitotic index was high and multinuclear giant cells were evident. The stroma contained collagen with erythrocyte extravasation. The tumor border was infiltrative. Immunohistochemically, the tumor cells were positive for SMA and negative for CK, desmin, CD34, S100, ALK, and β-catenin. Molecular detection demonstrated evidence of USP6 gene rearrangement in this tumor. Based on the findings, we diagnose this case as intramuscular NF. At 48 months after the initial surgery, the patient had recovered well with no evidence of recurrence or metastasis.
Conclusions: In this study, the diagnosis of intramuscular NF with a relatively large tumor was confirmed by molecular detection of USP6 gene rearrangement.