Desmoplastic small round cell tumor with atypical immunohistochemical profile and rhabdoid-like differentiation

Figure 1 Gross and microscopic feature of abdominal mass. A, B: Gross examination showed an encapsulated and lobulated soft tissue mass with attached omental tissue; C-F: The small round cell tumor possessed round to oval hyperchromatic nuclei with inconspicuous nucleoli and scant cytoplasm. Focal tumor cells with enlarged nuclei, open chromatin and prominent nucleoli, as well as rhabdoid differentiation were noted. Focal tumor necrosis and hemorrhages were present (HE staining).

Figure 2 Immunohistochemical and ultrastructural features of abdominal mass. Tumor cells exhibited strong diffuse cytoplasmic vimentin expression (A), diffuse membranous CD56 expression (B), cytoplasmic and membranous desmin expression (C), and nuclear INI-1 reactivity (SNF-5/BAF47) with tumor cells and non-neoplastic cells. Electron microscopy (E, F) showed closely apposed tumor cells with irregular nuclear outlines and heterochromatin. There were intermixed tumor cells with aggregates and whirls of intermediate filaments (arrows) that displaced the nuclei and entrapped organelles.