A 34-year-old woman presented with persistent abdominal pain. Radiologic examination demonstrated a 9.5 cm tumour in the sigmoid colon, causing partial bowel obstruction. The tumour was excised.

Pathologic Findings The specimen shows a multinodular neoplasm present in the wall of the bowel and diffusely infiltrating muscularis propria. There is focal mucosal ulceration and there is also multifocal necrosis deeper within the tumour. There is no evident mucosal origin. The tumour consists of sheets of rounded cells with limited amounts of pale cytoplasm and vesicular nuclei with small nucleoli. There are readily identified mitotic figures. Immunohistochemistry shows multifocal membranous positivity for CD99, but this is much less diffuse in extent than one would expect to see in Ewing sarcoma and the nuclear morphology would not fit well with the latter. In addition, there is strong and diffuse nuclear positivity for WT-1, while S-100 protein and HMB45 are negative. Fluorescence in situ hybridisation demonstrated CIC gene rearrangement.

ROUND CELL SARCOMA WITH CIC-DUX4 GENE FUSION