Contributed by Pooria Khoshnoodi, MD, and Ivy John, MD
A 50-year-old female with no significant surgical history presented with a left posterior thigh mass. Magnetic resonance imaging studies demonstrated a 3.0 cm peripherally enhancing, lobulated mass that was centered in the subcutaneous tissue of the left distal thigh.
Gross examination revealed a 3.7 x 3.0 x 2.1 cm well-defined, tan-yellow, focally hemorrhagic, lobulated mass involving the subcutaneous tissue (Fig. 1).
The tumor was composed of diffuse sheets of round to epithelioid cells with clear cytoplasm, vesicular chromatin, and prominent nucleoli arranged predominantly in lobules divided by fibrotic stroma (Figs. 2A, 2B, 2C and 2D). A minor spindle cell component and focal myxoid areas was identified. Increased mitotic figures (41/10 HPF) and tumor necrosis were noted. Immunohistochemistry showed multifocal positivity with CD99 (Fig. 3A) and WT1 (Fig. 3B); while essentially negative for LCA, CD38, MUM1, AE1/3, PANCK, STAT6, ERG, CD34, CD31, desmin, S100 and SOX10.
Next generation sequencing (NGS) studies performed on the paraffin-embedded tissue, demonstrated an in-frame fusion between genes CIC exon 20 (NM_015125) and DUX4 exon 1 (NM_033178).