Malignant rhabdoid tumors (MRT) represent a distinct group of aggressive tumors usually occurring in infancy involving a variety of anatomic locations including the kidney, brain, soft tissue and liver. The molecular hallmark of these tumors is the presence of deletions and/or mutations of SMARCB1 or rarely SMARCA4 that lead to the absence of INI1 or BRG1 proteins, core components of the SWI/SNF chromatin remodeling complex.
MRT of the liver are rare tumors that comprise less than 5% of malignant pediatric liver tumors. They are currently diagnosed on the basis of documenting both INI1 negative immunoreactivity and rhabdoid morphology. In the absence of molecular testing, MRT lacking classic rhabdoid morphology are currently classified as small cell undifferentiated hepatoblastoma (HB). Here we report a case of MRT initially classified as a INI1 negative small cell HB. Whole Exome Sequencing with Transcriptome subsequently showed homozygous deletion of SMARCB1 on chromosome 22q11.23 with evidence of loss of expression of the gene, establishing the diagnosis of MRT. As molecular testing of these tumors is performed on a more routine basis, this case illustrates that the INI negative small cell undifferentiated variant of HB actually represents MRT, a tumor unrelated to HB.
