Carcinosarcoma with rhabdoid features of the urinary bladder in a 2‐year‐old girl: Possible histogenesis of stem cell origin

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Abstract
A case of carcinosarcoma of the urinary bladder in a 2-year-old girl is reported. The tumor, measuring 34 × 20 × 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 dis-tinct 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, indicat-ing 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.