Paraffin sections show a densely hypercellular tumour composed of small cells with round and oval hyperchromatic nuclei and a small amount of pale cytoplasm. These are arranged in diffuse sheets within a stroma containing myxoid/mucinous material. Poorly formed cartilage is noted in one area. Tumour cells also form perivascular pseudo-rosette like structures around viable blood vessels. Frequent mitotic figures are identified and there is extensive tumour necrosis.
Immunohistochemistry shows strong membrane staining in tumour cells for CD56, patchy staining for CD99 (MIC-2) in a membranous and peri-nuclear pattern, and patchy staining for desmin, smooth muscle actin (SMA) and neurone specific enolase (NSE). No staining for GFAP, synaptophysin, TTF-1, tyrosinase or a range of cytokeratins (AE1/AE3, CAM5.2; CK7, CK20, CK19, CK5&6) is seen in tumour cells.
The features are of an undifferentiated malignant tumour. The myxoid stromal material and focal poorly formed cartilage suggest myxoid chondrorsarcoma. Immunostaining for CD56, NSE and CD99(MIC2) is consistent with this diagnosis.