Carcinosarcoma with rhabdoid features of the urinary bladder in a 2-year-old girl: possible histogenesis of stem cell origin.

A case of carcinosarcoma of the urinary bladder in a 2-year-old girl is reported. The tumor, measuring 34 x 20 x 18 mm, was located in the peri-trigone area of the urinary bladder with polypoid features. Histologic examination revealed transitional cell carcinoma at the tumor surface with downward invasion. Concurrently, a sarcomatous area was found beneath the carcinoma, with these two different malignant components sharing on apparent transition without distinct boundaries. Sarcomatous components included immature round cells focally showing rhabdoid features. No rhabdomyomatous component was observed. Immunohistochemistry disclosed vimentin and cytokeratin-double positive cells at the transposition between carcinoma and sarcomatous components. In addition, ultrastructural analysis revealed that the epithelial cells had a distinct junctional complex, and the sarcomatous cells occasionally had a meshwork of cytoplasmic intermediate filaments, indicating bidirectional cytodifferentiation to epithelial and mesenchymal elements. The extremely young age at which this case of carcinosarcoma occurred suggests that the tumor may be of mesodermal stem cell origin.