Case report: Large-size intramuscular nodular fasciitis, a challenging histopathologic diagnosis confirmed by molecular detection of USP6 gene rearrangement: Case report and literature review

Abstract

The intramuscular subtype of nodular fasciitis (NF) is rare with lesions normally not more than 2 cm in size and characterized by pseudosarcomatous morphology. We report a 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 actin (SMA) and negative for cytokeratin, desmin, H-Caldesmon, CD34, S100, ALK, and β-catenin. Fibrosarcoma was highly suspected by histopathological and immunohistochemical examination. Molecular detection demonstrated evidence of ubiquitin-specific peptidase 6 (USP6) gene rearrangement 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. Large-size intramuscular NF is very rare and easily overdiagnosed as malignant tumor due to its obvious pseudosarcomatoid pathological features. USP6 gene rearrangement detection can effectively avoid this major misdiagnosis.

Keywords: USP6; gene rearrangement; intramuscular; large-size; nodular fasciitis.

FIGURE 1

MRI revealed an irregularly well-defined nodule at the outer edge of the middle humerus. (A) The nodule (the arrow) had slightly enhanced signal intensity on T1-weighted imaging. (B) On T2-weighted imaging, there was significantly enhanced heterogeneous high signal intensity (the arrow).

FIGURE 2

Histopathological findings of the intramuscular tumor. (A) Tumor cells infiltrating into the surrounding skeletal muscle (HE× 100). (B) Tumor cells arranged in an interlaced pattern (HE×40). (C) In the scarce cell area, there were microcystic changes (the arrow) (HE× 100). (D) Mitotic figures were high (the arrow) (HE× 400) (E) Multinucleated cells were conspicuous (the arrow) (HE× 400). (F) The tumor showed erythrocyte extravasation (the arrow) in the stroma (HE× 400).

FIGURE 3

(A) Immunohistochemical staining. Tumor cells were strongly immunoreactive for SMA. (×100). (B) The multinuclear giant cells were positive for CD68 (× 100). (C) Ki-67 proliferation index was 10% (×100). (D) FISH illustrated rearrangement of the USP6 gene locus 17p13 using a break-apart probe set, with separation of red and green signals (the arrow).