Case 4: Malignant mesenchymal neoplasm (NOS) with rhabdoid features

The neoplasms is a solid-appearing moderately cellular spindle cell neoplasms with abundant intercellular collagen deposition. The tumor is moderately vascular, but without the typical “staghorn vessels”. A number of the blood vessels are hyalinized. In some field the cells have “rhabdoid” features with abundant eosinophilic cytoplasms and eccentrically displaced nuclei Only occasional mitoses are seen. By immunohistochemistry, the tumor cells show strong membrane pattern immunoreactivity for CD99. The INI-1 (BAF47) stain shows diffuse nuclear immunoreactivity consistent with retained expression. Stains for smooth muscle actin, CD31, and CD34 highlight the rich vascular network, although the cells are negative. The tumor is also negative for BCL-2, synaptophysin, GFAP, and epithelial membrane antigen (EMA). The Ki67 labelling index is low (3%). Ultrastructural analysis performed from paraffin material did not show any evidence of meningothelial differentiation. Differential diagnosis: Solitary fibrous tumo Based on pure morphology this diagnosis was favored. In support of this interpretation, the CD 99 showed diffuse embramnous immunoreactivity. However against this diagnostic possibility was the lack of either CD34 or BCL-2 immunoreactivity. r