Extrarenal primitive malignant tumor with rhabdoid features: fine-needle aspiration cytology, immunocytochemistry, and electron microscopy of a case.

We describe a malignant pelvic tumor with rhabdoid-like features that arose in the sacroiliac region of a 15-year-old girl. Fine-needle aspiration (FNA) cytology showed large cells with glassy cytoplasm, globular hyaline cytoplasmic inclusions, and prominent nucleoli. Immunocytochemical methods applied to FNA material revealed immunoreactivity for cytokeratin, epithelial membrane antigen, and vimentin; the cells failed to react with antibodies to desmin. This distinct appearance and phenotype was most consistent with a diagnosis of extrarenal malignant rhabdoid tumor. Electron microscopy confirmed this interpretation, showing whorled filamentous aggregates in the cytoplasm of the tumor cells.