Résumé
Malignant rhabdoid tumor is a distinct renal tumor in the pediatric age group. It was originally described as a rhabdomyosarcomatoid variant of Wilms tumor. However, subsequent studies failed to confirm myogenous differentiation, so it is now considered to be a distinct and unique type of highly malignant tumor, histogenetically unrelated. Although extrarenal forms of this tumor are rare, several examples have been described in other sites, especially the liver, prostate, paravertebral area, urinary bladder and soft tissue. We experienced a case of malignant rhabdiod tumor located in the intraabdominal cavity in a 10 month-old boy. Smear of peritoneal fluid showed round, polygonal and irregular shaped cells with large nuclei, ample cytoplasm containing Jight pink "to purple cytoplasmic inclusions, and one or a few prominent nucleoli. Immunocytochemistry revealed positivity to cytokeratin, epithelial membrane antigen and vimentin, and negativity to desmin and neuron-specific enolase. These distinct cytologic appearance and immunophenotypes were most consistent with a diagnosis of extrarenal malignant rhabdoid tumor. The cytoplasmic inclusions were correlated with eosinophilic inclusions seen in histologic section and electron microscopy confirmed this interpretation, showing filamentous aggregations in the cytoplasms of the tumor cells.