Carcinoma of the urinary bladder: a review of its diverse morphology.

The microscopic appearances of carcinomas of the urinary bladder are reviewed, with emphasis on patterns that have been highlighted only recently or account for major problems in differential diagnosis. Approximately 80% of bladder cancers are of urothelial (transitional cell) type. The major new information pertaining to that category is the appreciation of the extent to which they may have unusual patterns that, on occasion, lead to their misinterpretation as benign. The homology between some of these variants and benign epithelial abnormalities is emphasized and helps in understanding them: nested carcinomas and von Brunn's nests, small tubules and nephrogenic adenoma, carcinomatous glands and cystitis glandularis, cystic carcinomas and cystitis cystica. Other peculiar patterns seen in urothelial carcinoma include silt-like spaces, clefts, and angulated nests with spikes. Differences in opinion have been apparent in recent contributions concerning carcinomas of the bladder with a prominent component of spindle cells (sarcomatoid carcinomas) and whether they represent a category distinct from carcinosarcoma. Irrespective of this academic distinction, the potential for some epithelial-derived tumors of the bladder to be dominated by or even have an exclusive component of spindle cells is important. Three other major categories of primary bladder cancer are squamous cell carcinoma, adenocarcinoma, and undifferentiated carcinoma. The differences in the frequency of subtypes of adenocarcinoma originating in the bladder mucosa and urachus has recently been emphasized, as has the spectrum of undifferentiated carcinomas. In some bladder cancers, the stroma has unusual features. These are carcinomas with pseudosarcomatous stroma, osseous or cartilaginous metaplasia, or osteoclast-type giant cells. The diversity of appearances of bladder cancer has important implications in differential diagnosis.