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 rep orted. The tumor, measuring 34 x 20 x 18 mm, was located in the peri-trigon e area of the urinary bladder with polypoid features. Histologic examinatio n revealed transitional cell carcinoma at the tumor surface with downward i nvasion. Concurrently, a sarcomatous area was found beneath the carcinoma, with these two different malignant components sharing on apparent transitio n without distinct boundaries. Sarcomatous components included immature rou nd cells focally showing rhabdoid features. No rhabdomyomatous component wa s observed. Immunohistochemistry disclosed vimentin and cytokeratin-double positive cells at the transposition between carcinoma and sarcomatous compo nents. In addition, ultrastructural analysis revealed that the epithelial c ells had a distinct junctional complex, and the sarcomatous cells occasiona lly had a meshwork of cytoplasmic intermediate filaments, indicating bidire ctional cytodifferentiation to epithelial and mesenchymal elements. The ext remely young age at which this case of carcinosarcoma occurred suggests tha t the tumor may be of mesodermal stem cell origin.