Immunohistochemical study as a tool in differential diagnosis of pediatric malignant rhabdoid tumor.

Malignant rhabdoid tumors (MRTs) are aggressive childhood neoplasms, occurring mainly in the kidney and brain. We describe 2 unusual cases of extrarenal and noncranial location (liver and soft tissue with dissemination) mimicking hepatoblastoma, neuroblastoma or Ewing sarcoma. Both cases revealed a polyphenotypic profile, combined with cytokeratin, vimentin, and CD99 expression. INI1/BAF-47 showed negative protein nuclear expression in both cases, suggesting a diagnosis of MRT. An extensive immunohistochemical panel was performed to exclude pediatric tumors reminiscent of MRT. The genetic studies failed to detected MYCN amplification, 11q23 deletion, and EWS break-apart positivity. No alterations of 22q integrity were demonstrated with the probes used for the study (N25 Di George/22q11.2, 22qter, and EWS/22q12). We discuss the differential diagnosis in pediatric polyphenotypic tumors (Wilms tumor, neuroblastoma, desmoplastic small round cell tumor, and Ewing sarcoma). Analysis of INI1/BAF-47 expression can offer important clues in the diagnosis of pediatric tumors with rhabdoid phenotype. The integration of clinical, morphologic, immunohistochemical, and genetic data is required to approach a correct diagnosis of pediatric tumor in unusual location with atypical or undifferentiated morphology.