PMC9894875_01
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).
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F1
T2-weighted imaging (Figure 1B). The MRI findings suggested the possibility of fibromatosis. The patient underwent surgery for mass excision.
Histopathological findings of the intramuscular tumor. (A) Tumor cells infiltrating into the surrounding skeletal muscle (HEx 100). (B) Tumor cells arranged in an interlaced pattern (HEx40). (C) In the scarce cell area, there were microcystic changes (the arrow) (HEx 100). (D) Mitotic figures were high (the arrow) (HEx 400) (E) Multinucleated cells were conspicuous (the arrow) (HEx 400). (F) The tumor showed erythrocyte extravasation (the arrow) in the stroma (HEx 400).
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F2
Microscopic examination showed that the tumor was relatively well demarcated, but in the focal, it was infiltrating the adherent muscle (Figure 2A). It was highly cellular and composed of fibroblasts and myofibroblasts arranged in bundles with an interlaced pattern (Figure 2B). Part of the lesion was discohesive and myxoid with a microcystic appearance (Figure 2C). Some tumor cells had mild atypia with active mitotic counts (the average mitotic counts up to 8 per 10 high-power fields, Figure 2D) and mingled conspicuous multinuclear or atypical giant cells (Figure 2E). Erythrocyte extravasation was easily seen on a background of collagenous stroma (Figure 2F), with some hemosiderin deposition and scattered lymphoplasmacytic cells infiltration.
(A) Immunohistochemical staining. Tumor cells were strongly immunoreactive for SMA. (x100). (B) The multinuclear giant cells were positive for CD68 (x 100). (C) Ki-67 proliferation index was 10% (x100). (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).
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F3
The tumor cells were only positive for smooth muscle actin (SMA) (Figure 3A), negative for cytokeratin, desmin, H-Caldesmon (see supplement data), CD34, S100, ALK, and beta-catenin. The multinuclear giant cells were positive for CD68 (Figure 3B)). The average rate of ki67 positivity was 10% (Figure 3C). The FISH results revealed evidence of USP6 gene rearrangement given the division of green and red signals (Figure 3D). The average percentage of USP6 split signals was 40%.